The digestive tract of the 18 mm embryo horse

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Content THE DIGESTIVE TRACT
OF THE 18 mm. EMBRYO HORSE
A Thesis
Presented to the Department of Zoology
University of Southern California
In partial fulfillment
I Degree not
of the granted; course
Requirements for the
Degree of Master of Arts
work incomplete :
By
Inez Wilber
June,1931
UMI Number: EP67061
All rights reserved
INFORMATION TO ALL USERS
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'Dlssertfftioni: PbWkNng
UMI EP67061
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PREFACE
The author wishes to express her sincere thanks and
appreciation for the very material aid and encouragement
given her by Dr. Bruce Magill Harrison, under whose guidance
this problem was developed.
The material on which this investigation was based
was obtained through the courtesy of Miss Louise Mohn and
Miss Marion Vary.
It is a pleasure to the author to acknowledge her in
debtedness to Mrs. Ruth Her, to Mrs. Wallace P. Wood, and
to Mr. Robert M. Parker*
T h is the sis y w ritte n u n d e r the d ire c tio n o f the
candidate^s F a c u lty C o m m itte e a n d approved by
a ll its m em bers, has been presented to a n d ac
cepted by the C o u n c il on G rad uate S tu d y a n d
Research in p a r tia l fu lfillm e n t o f the re q u ire
m ents fo r the degree of
Master of Arts
S e c re ta ry
D e a n
F a c u lty C o m m itte e
Professor H^rlso^
C h a irm a n
P.rafe-ssjor—Eo-ssl-Qr.....
Professor Roberts
11
THE DIGESTIVE TRACT OP THE 18 mm.
EMBRYO HORSE
IHTRODUCTIOH
Our present knowledge of the development of the horse
Is far from being complete. In fact, there Is comparatively
little literature and those who have worked most In this
field are agreed that a thorough investigation into the de
velopment of the horse Is yet to be made.
In this investigation the 18 mm. embryo alone has been
considered, and reference Is made to other stages only by
way of making comparison. The digestive tract of this stage
has been modeled with the desire to get an accurate concep
tion of the tract as a whole, and to present the facts con
cerning the relative stages of advancement In Its various
differentiating regions and dlvertlculae.
Attention has been centered on the plan of development
of this particular embryonic stage, and some evidence has
been obtained which at least has the Interest of newness.
Because the digestive tract Is so Important a part of an
embryo, and because the tract gives rise to derivatives so
essential In the morphology and physiology, the author hopes.
In presenting the results of this Investigation, that she
has. In some small part, been able to add to the few known
facts regarding the embryology of the genus equldae.
CONTENTS
INTRODUCTION
REVIEW OF LITERATURE............. 2
MATERIAL AMD METHODS ................................... 4
ESOPHAGUS................................................ 6
STOMACH................................................. 11
SMALL INTESTINE............................ 16
Duodenum...........................................16
Bile and Pancreatic Ducts .......... 16
Liver .......................................19
Jejunum and Ileum ........................... 20
Yolk Stalk.................................... 23
LARGE INTESTINE.........................................24
Caecum and Appendix................................24
Great Colon................... 26
Floating Colon . 26
Rectum and Cloaca ............. ..... 26
Allantois............ 27
Anal Region........................................28
Mesentery . ..................... 29
BLOOD VESSELS...........................................30
SUMMARY.......... 32
BIBLIOGRAPHY............... 35
INDEX TO ILLUSTRATIONS . 39
INDEX TO ABBREVIATIONS................... 42
EXPLANATION TO PLATE 2 .................................45
EXPLANATION TO PLATE 4 ................... 46
REVIEW OP THE LITERATURE
The ancestry of the equldae has absorbed the interest
and investigation of several men. Scientific breeding of
horses dates back to ancient times, practised and perfected
in many countries; handsomely illustrated descriptions of
every type of soliped are available; the anatomy of the adult
horse has been well worked out by a few, but a satisfactory
treatice on the embryological development of the horse is
not to be found.
Hausmann (*40)^ whose work is the earliest in this
particular field described the developmental stages during
the earlier weeks. However, it has been discredited as
being inaccurate.
Huxley (’76)2 devoted much of his energy to the ances
tral development of the horse, as it bears upon the problem
of evolution.
Bonnet (’89)^ described the foetal membranes of the
21-day old embryo, and this work is now considered among
the most reliable, but his embryo was much younger than the
one described by the author.
Martin (’90)'^ also described the 21-day embryo which
differed slightly in size and shape from that of Bonnet’s.
Though Ewart (’15) criticizes Martin’s conclusions,
lEwart, James C®ssar, Studies in the Development of the
Horse. Trans. Roy. Soc. Edin. 1916-17, Vol. 51, Part 2,
p. 287-330.
2jbid.
^Ibid.
4lbid.
3
Bonnet’s findings have been used by such serious embryolo
gists as Jenkinson (’13) and McEwen (»23). Ewart has made
contributions of worth, and used extreme caution in time
of breeding and autopsying, producing, as nearly as is pos
sible, an authentic 21-day embryo from which he constructed
a model. The model is well interpreted by Robinson (’16).
Dadd (’57),Chauveau■(’08), Vaughn (* 17 ), Davison (» 23),
and Sisson (*27), concerned themselves chiefly with the
anatomy of the adult horse. They offer little or nothing
on the development of the horse.
In general consideration of embryology, very satisfac
tory descriptions of the digestive tract are given by the
following authors: Marshall (’93), Foster (*98), Jenkinson
(*09), McMurrich ($15),^ Prentiss (*15), Jordan (*17),
McEwen (* 23 ), Arey (* 24 ), Shumway (* 27), and Dodds (* 29).
Lillie (*08), centers his attention on the various stages
of the chick’s development. Patten (*27) wrote on the
early development of the pig, as also did Lowrey (*12).
Dean (*06) has presented an interesting article on the
chimaeroid fishes. Huber (*15) has done excellent work on
the albino rat. Reighard and Jennings (*01) contributed
a text on the anatomy of the cat. Especially noteworthy are
the papers by Reese (*08), (*10), and (*26); and the one by
Jordon (*17) dealing with the embryology of the loggerhead
turtle.
MATERIAL AND METHODS
In Washington state are to he found roaming hands of
small wild horses, and it is believed that these are not
the true wild horses, but descendents of domesticated
animals brought west at an early date and, for some reason,
turned out to their own devicesIndians drive these
horses to slaughter houses and the meat is distributed to
surrounding fox farms. An observing eye noticed that the
mares are often with foal. In 1929 eight embryos, in uteri,
were procured and sent to Miss Marion Vary, who later turned
them over to Miss Louise Mohn.
Miss Mohn prepared the 18 ram. horse embryo for micro
scopic study, upon which this work has been based. Although
her own field of research dealt only with the pharyngeal
derivatives, she quite graciously left all material with
the department, where it is now, and no'doubt will continue,
to prove of inestimable value for other embryologie in
vestigations .
The writer used the micro-projector in the work of
reproducing all cross-sections. Outlines of the cavities
of the digestive tract and principal arteries were drawn on
straw board 2 mm. thick. These drawings represent cross
sections of embryonic tissue 80 micra thick. These out
lines were perforated by means of a sewing machine, the
desired areas were punched out along the perforations and
^Ewart, James Cossar, The Multiple Origin of Horses and
Ponies. Smith. Inst. Annual Report, 1904, p. 437-455.
put together with melted paraffin. Accurate relations be
tween cross sections were made possible by measurements
taken at the level of every cross section. Because of the
size, the model was built in two parts, the lower represent
ing slides three to six and the upper, slides seven to nine,
(Pig. 6 a). The entire model was painted with a mixture of
asphalt and bees-wax, making a structure less fragile. Since
a mass of black tubes coiling in and out was found to be
confusing it was decided that some means of differentiating
between the digestive and blood vascular systems must be
used. Hence, the blood vessels were left black and the di
gestive canal was painted with chrome yellow powder mixed
with melted paraffin. The model thus gives a general idea
of the relationships of the entire digestive tract.
Photographs were taken of the model at various angles,
and camera lucida drawings were made of the cross sections
thought to be of greatest value in interpreting descriptions
found in the text.
6
ESOPHAGUS
The esophagus of the IS mm. embryo horse is found to
be a hollow tube 4.50 mm. in length, extending from pharynx
to stomach. It is part of the gut which, relatively con
sidered, does not undergo much enlargement. Its length in
creases as the heart, stomach, and diaphragm migrate caudad.^
According to Kingsley,^ the length of the adult esophagus
varies in different animals with the length of the neck,
being short in reptiles and reaching an extreme in birds.
As given by Chauveau,^ for the adult horse, it is a "long,
cylindrical, narrow, membranous canal, a little wider below
its commencement......... In its course it shows several
changes of direction. The esophagus of the adult horse be
gins at the pharynx and communicates with it by means of
the posterior opening situated above the glottis. It soon
after regains its position above the trachea, passes over
the base of the heart, and reaches the opening in the right
pillar of the diaphragm into the smaller curvature of the
stomach by the cardiac orifice." Pig. 2 taken from Sisson
illustrates the above description in the adult horse.
The esophagus of the 18 mm. embryo horse. (Pig.12),
begins in the median plane just dorsal to the larynx, between
that tube and the notochord. Posterior to its cephalic
end, 1.16 mm., it inclines slightly to the right of the
iDodds, Gideon S., Essentials of Human Embryology. J. Wiley
and Sons, N.Y., 1929.
^Kingsley, J. S., Comparative Anatomy of Vertebrates.
P. Blakiston’s Sons and Co., Phila., 1917.
^Chauveau. A., Comparative Anatomy. D. Appleton and Co.,
N.Y., 1908.
trachea, (Pig. 16), where it continues downward to the region
of the bronchi (Fig. 18), at which location, 2.38 ram.
farther caudad, it again assumes a median position and con
tinues between the two lobes of the lungs (Pig. 19). Pos-
teriorwards 1.96 mm., the esophagus turns gradually to the
left, reaches the hiatus of the diaphragm, and passing through,
terminates at once, a little to the left of the median plane,
in the cardiac orifice of the stomach (Fig. 22). As in the
adult horse, when viewed from the dorsal plane, the course
of the esophagus is,according to Sisson,^ downward and back
ward until it enters the thorax and then passes upward to
reach the dorsal face of the trachea. Then to the root of
the lung it passes somewhat upward to its termination.
The esophagus of the 18 mm. horse tapers gradually for
some distance from the pharynx (Figures 10 and 11) and then
continues for a greater distance as a cylindrical canal of
almost uniform diameter (Pig. 3) which is a condition ob
served on the alligator by Reese.^ The lumen is open
throughout. In cross section (Pig. 12) the lumen is boat
shaped and compressed dorso-ventrally. For a distance of
.28 mm. posteriorwards the lumen continues to be compressed,
but becomes less extensive from side to side. Prom here
backwards there is less lumen. Caudad, for a distance of
1.42 mm. there is aq slight enlargement which persists .22 mm.
^Sisson, Septimus, Anatomy of Domestic Animals. W. B.
Saunders and Co., Phila. and London, 1921.
^Reese, Albert Moore, The Development of the Digestive Canal
of the American Alligator, Smith. Misc. Coll., Vol. 56.
No. 11. Pub. No. 1946, 1910.
8
The lumen then assumes an oval shape, in cross section,
slanting downwards from right to left. Farther posterior it
again rights itself and becomes, once more, slightly com
pressed dorso-ventrally. But, .06 mm. farther caudad, it
resumes a slant from above downward, or from the right side
to the left. At a length of 1.26 mm. the lumen appears de
finitely rounded in cross section (Fig. 14). Except for a
slight turning to the right, .12 mm. farther caudad, and a
dorso-ventral compression farther back .14 mm., it remains
uniform. The lumen appears, in cross section,to be closer
to the stomach when it has reached a length of 1.76^mm. At
a point 1.92 mm. from its anterior end the esophagus comes
to occupy a position a trifle to the right of the trachea.
Likewise, at 2.42 mm. the lumen is found to be compressed
laterally, but again loses this compression .3 mm. farther
on. At the bifurcation of the trachea (Fig. 17), a distance
of 3.20 mm. from the beginning of the esophagus, the lumen
is directly posterior to the bronchus. It appears egg-
shaped in cross section, the pointed end directed posterior
wards. The uniform roundness is resumed at 4.12 mm. From
here on caudad it remains very small, becoming small Indeed
at a distance of 4.38 mm. Here, the lumen elongates and is
narrowed directly caudad, maintaining a position between
the narrowing lung buds. The lumen of the esophagus con
tinues to elongate, reaching a length of 4.50 mm., where,
in a position posterior to the lung buds, it lengthens out
to the left and becomes continuous with the lumen of the
stomach (Pig, 2,2).
As it passes down the neck, the esophagus is accom
panied by the trachea to the point where the latter bifur
cates. Where the esophagus separates from the pharynx, it
lies anterior to the centrum of the cervical vertebrae, pos
terior to the trachea, and median to the blood sinuses.
Entering the mediastinum, the anterior aorta lies ventral
to the trachea, as well as lateral. As the region of the
heart is appraoched the blood vessels are lateral to the
trachea and esophagus. Below the bifurcation of the trachea
(Fig. 20) the bronchi lie ventral to the esophagus, while
the lung buds are lateral, and the dorsal aorta is posterior,
The dorsal aorta remains in this position through the medias
tinum, while the lung, maintains m ventral and lateral posi
tion. As the esophagus appraches the diaphragm the lungs
become dorsal to it and the liver and stomach lie ventral.
The esophageal wall is practically uniform in thickness
throughout its entire length, though tapering from its
cephalic end to its termination at the cardiac orifice of
the stomach. The cell structure, as seen on the slides, is
made out with difficulty. The deeply staining entodermal
tissue is differentiated from the lightly staining mesoder
mal wall. The entodermal lining, while lightly staining,
is also characterized by the direction of its nuclei which
lie at right angles to the lumen. The nuclei of the meso
dermal cells are oval in outline and lie somewhat parallel
to the long dimention of the cell. The mesodermal tissue
of the wall is two or three times the thickness of its
10
entodermal lining. In cross section, the arterial branches
are seen to be in almost continuous communication with the
outer wall of the esophagus.
11
STOMACH
The stomach of the 18 mm. embryo horse, like the adult
horse of Vaughn,^ (Pig. 2), is a dilatation of the alimen
tary canal continuous with the esophagus and small intestine.
It lies in the left hypochondrium, transverse to the long
axis of the body of the embryo, and is held to the left
side by the esophagus. In the model (Fig. 5) and in the cross
section (Pig. 22) the embryonic stomach resembles, in a
general way, that of the adult, bearing the appearance of a
tube bent on itself. Sisson^ describes the adult stomach as
a "sharply u-shaped sac, relatively small, in the dorsal
part of the abdominal cavity, behind the diaphragm and liver.
...... When distended it resembles a bent tube with two
lateral dilatations, divided by a central constriction into
a left cardiac and a right pyloric portion. The cardiac is
larger and called the greater cul-de-sac, or fundus; and the
right, the lesser cul-de-sac, or pylorus."
The embryonic stomach is slightly dilated along its
convex border so as to form the suggestion of the two cul-
de-sacs, right and left. At this 18 mm* stage the stomach
has rotated on its long axis, the dorsal side has grown
more rapidly than the ventral, and the organ has been crowded
to the left side of the coelom. This condition is comparable
1Vaughn, Strangway’s Veterinary Anatomy. Chicago Medical
Book Co., 1917.
^Sisson, Septimus, op.cit.
12
to that reported by Dadd^ for the adult horse. The change
of position in the human, according to Prentiss,^ is com
pleted at the 12 mm. to 15 mm. stage. The caudal, or
pyloric end, of the stomach is now ventral and to the right
of its cardiac, or cephalic, end. The whole organ (Fig.21)
thus extends obliquely across the peritoneal cavity from
left to right.
The two curvatures of the stomach found in the adult
horse are present in the 18 mm. stage and indicate a di
vision into two portions, or surfaces, the parietal and vis
ceral. The parietal, according to Sisson,^ is that surface
directed upward to the left, which in the adult lies against
the liver and diaphragm. The visceral is also convex, and
faces in the opposite direction from that of the parietal,
and in the adult is adjacent to the large colon, pancreas,
small colon, small intestine, and great omentum. In the
18 mm. embryo the visceral surface is adjacent only to the
pancreas and great omentum.
The borders between the surfaces are known as curva
tures. The lesser is a concave curvature Into which the
esophagus is inserted, while the greater is convex inferiorly
and gives attachment to the great omentum. The latter, in
the 18 mm. embryo, and as given by Sisson^ for the adult,
^Dadd, Geo. H., Anatomy and Physiology of the Horse. John
P. Jewett and Co., 1867.
^Prentiss, Chas. Wm., Textbook of Embryology. W. B. Saunders
and Co., Phila. and London, 1915.
^Sisson, Septimus, op.cit.
4Ibid.
13
"is extensive and is first directed dorsally, curves over to
the left extremity then descends, passes to the right, crosses
the median plane, and curves upward to end at the pylorus.”
In the embryo the lesser curvature is approximately one-half
the length of the greater.
The stomach of the 18 mm. stage like that of the adult,
possesses two extremities, a left, rounded in form, which
in the adult measures 8 to 10 inches from the cardia ; and
the right, or pyloric, which is continuous with the duodenum.
According to Jordan and Kindred,^ speaking of the embryonic
state in the human, "the pyloric portion of the stomach in
the 10 mm. to 20 ram. stage is approximately one-half of the
entire organ." This is found to be the case in the 18 mm.
horse, as shown in the reconstructed model (Pig. 5).
A comparison between the 18 mm. embryonic stomach and
that of the adult horse shows much similarity in the con
dition of the cardiac and pyloric orifices. The cardiac,
in the lesser curvature, is the orifice into which the
esophagus is inserted, almost perpendicularly. No widening
is noticed at this orifice at either the 18 mm. or adult
stage, the lumen being even narrower than at any level above
it. "The pyloric orifice", according to Chauveau,^ "is
the larger aperture at the bottom of the right sac." It
is continuous with the duodenum (Fig. 3).
^Jordan, Harvey Earnest, and Kindred, James Earnest,
Embryology. D. Appleton and Co., N.Y., 1929.
^Ghauveau, A., op.cit.
14
In cross section, the lumen of the embryonic stomach,
when it first appears, is elongated laterally to the right.
Caudad, 100 micra it is curved on itself, somewhat con
stricted in the middle and slightly depressed dorso-ventrally.
The pointed end of the curve is directed to the left and the
broad end, downward to the right. 1.4 mm. caudad from the
appearance of the lumen, there is a change in shape and
position. Its outline, in cross section, is now irregulary
”T" shaped with the base directed obliquely upwards to the
right, and the right and left arms pointing down to the
right and up to the left, respectively (Pig. 20). The out
line of the lumen changes gradually, pointing directly up
to the approaching esophageal lumen. The arms of the "T"
widen into a club-like expansion. When the lumen becomes
continuous with that of the esophagus in the same cross
section (Fig. 22), it proceeds to decrease in size, and so
continues, until it entirely disappears at the left extremity.
In the early appearance of the lumen, in cross section, the
lower right hook-like portion of the broad end appears to
constrict off and becomes the duodenum (Pig. 26). The many
folds in the lining of the wall of the empty embryonic
stomach cause the reconstructed model to be irregular in
appearance.
In cross section, the shape of the outside limit of
the stomach is practically triangular, but soon becomes the
shape of a half moon, which shortly acquires a hook-like
projection which points down to the right (Pig. 19). Very
15
early in cross section, it takes on the characteristic out
line showing two borders, two curvatures, two surfaces, and
two extremities (Fig. 21). Caudad .44 mm. to the first
section through the stomach, the coelomic cavity appears,
widens caudally, and comes to surround the stomach, except
for the upper left third of its greater curvature, where,
due to its anatomical location the stomach has attached to
it the gastro-splenic ligament. The gastro-colic ligament
is also recognized in cross section (Fig. 21).
The stomach wall at its widest part reaches a thickness
of .38 mm. The cell structure, except for the nuclei, is
not visible on the thick glass slides upon which the sections
are mounted. The nuclei of the lining stain very deeply,
and the thickness of the entoderm is probably as much as
twice that of the esophagus. The gastric pits, which accord
ing to Prentissare indicated at 16 mm. in the human, are
not seen, neither are the cardiac glands, which in other
forms develop at 91 ram. The three layers of stomach muscle
are not differentiated at this stage, the entire stomach
wall appearing uniform in cell structure, the nuclei smaller,
more rounded, and more densely arranged than those of the
lining entoderm.
^Prentiss, C. W.. op.cit.
lé
SMALL INTESTINE
Duodenum. The duodenum of the 18 mm. embryo horse is
in general similar to that of the adult. It is short and
presents a dilatation at its origin resembling a miniature
stomach. The convexity of the duodenum lies in an opposite
direction to the convexity of the stomach. Like the adult
duodenum, it may be divided into three parts, anatomically
speaking, the first, an enlarged region, forms a "U" shaped
curve with its convex side directed to the right ; the second,
possesses a lumen which gradually becomes smaller caudad,
passes upward and slightly backward, which, according to
Sisson,! is a condition existing in the adult. The third
part crosses the median plane from right to left to join
the jejunum (Fig. 5).
Bile and Pancreatic ducts. In cross section, the ducts
of the liver and the pancreas are seen entering the duodenum
a short distance from the pylorus (Figures 27 and 28). Ac
cording to Prentiss,2 and Mentzer,^ the gall bladder is
normally absent in the adult horse. In the human, according
to Prentiss,^ the primitive hollow hepatic divert1culum
later differentiates into the gall bladder and larger biliary
ducts. It may be, as in some animals as the petromyzon and
pigeon,5 a gall bladder develops in early embryonic life to
Islsson, Septimus, bjp*d t .
^Prentiss, C. W., op.cit.
^Mentzer, S. H., Anomalous Bile Ducts In Man. J.A.M.A. Oct.
26, 1929, Vol. 93, No. 17, p. 1273.
4Prentiss, G. W. op.cit.
%entzer, S. H. , op.cit.
17
disappear in later development, for none is evident in the
horse at 18 mm. "Most animals", states Vaughn^, "are sup
plied with a gall bladder lodged in a fissure on the pos
terior surface of the liver in which bile collects during
the period of abstinence and from which it passes into the
intestinal canal when digestion commences .But in soli-
peds, in the elephant, and in hyrax, the secretion of bile,
though more active during digestion, is constant, there being
no reservoir for it.” Mentzer^ says that anomalous biliary
structures in man represent the normal in lower animals. He
reports 10% of his human cases in which there was a congeni
tal absence of the gall bladder.
A ”Y” shaped duct is seen close to the upper right side
of the stomach wall, the right and left branches extending
only a short distance into the right and left lobes of the
liver. This is the hepatic duct (Fig. 27). The lower por
tion of this duct becomes continuous with another which,
in previous cross sections is seen to be an open invagination,
the hepatic groove, into the right lobe of the liver, and
has farther caudad, undergone differentiation to become the
common bile duct (Fig. 27). This common duct of this embryo
horse slants downward through the intestinal wall to enter
the duodenum with the duct of the dorsal pancreas (Pig. 28).
In the human, according to Mentzer,^ the common duct some
times unites with the dorsal pancreatic duct at a short
1Vaughn, op.cit.
|Mentzer, S. H..op.cit.
^Ibid.
18
distance from the duodenum to form a ductus communis. Such
a condition is not found in the 18 mm. horse, nor is a
cystic duct identified.
In the 18 mm. horse the ventral and dorsal pancreatic
ducts are seen entering the duodenum at different'levels
(Fig. 27). A similar condition is reported for the human
by Prentiss.^ An intricate duct system is indicated in
cross section (Fig. 30), by the numerous lumina budding and
rebudding off from the pancreatic ducts. In the human at
9 ram.,according to Dodds,^ "the pancreas arises from the
duodenal part of the developing gut by two separate hollow
outgrowths.......One, the dorsal pancreas, arises as a
hollow outpocketing of the dorsal duodenal wall slightly
caudad to the hepatic diverticulum, and the ventral pancreas
develops in the inferior angle between the hepatic diver
ticulum and gut......... In the human the dorsal grows
faster than the ventral, and comes to form the greater part
of the pancreas The two unite at 20 mm..... Before
union, each has its own duct. Upon union, the duct of the
small ventral part, the duct of Wirsung, becomes the main
pancreatic duct, while the duct of the larger dorsal part,
duct of Santorini, becomes accessory, and may lose con
nection with the duodenum." Both ducts persist in the adult
horse, according to Arey.^ The epithelium of the ducts
Iprentiss, C. W..op.cit.
2Dodds, G. S..op.cit.
^Arey, Leslie Brainard, Developmental Anatomy. W. B. Saunders
Co., Phila. and London, 1924.
may be identified under high pov/er and is well differentiated
from the more lightly staining portion of the gland, which
is the supporting portion. Alveoli, if present, are not dis
cernible at this stage, nor is any trace of the isles of
Langerhans. This absence is to be expected, however, since
in other closely related vertebrate forms, these isles do
not appear until 55 mm.l
Liver. At 18 mm. the liver already occupies the larger
portion of the body cavity, although in the human, according
to Dodds,2 it doesn’t reach its maximum size until 31 mm.
The 18 mm. horse liver exhibits no characteristic structure
as does that of the adult horse. The writer is unable to
identify bile capillaries at this stage, only the hepatic
and common duets being determined. This condition is com
parable to the human embryo previous to 22 mm., at which
stage, according to Prentiss, bile capillaries develop,
spreading inward, from the hepatic duct. In the human, the
hepatic cells are arranged in solid cords at the 10 mm.
stage.^ In the 18 mm. horse the hepatic cells appear to
pack the gland in no definite arrangement, the cords of
cells not yet radiating from the central veins. Until
20 mm., according to Prentiss,^ in speaking of the human,
the portal vein alone supplies the liver, the hepatic ar
tery being first in relation with the hepatic duct.
1Prentiss, C. W. op.cit.
Dodds, G. S. op.cit.
^Prentiss, C. W. op.cit.
4Ibid. ---
:2o
In the human, the anlage of the liver arises as a hol
low outgrowth of entoderm from the mid-ventral line of the
duodenal part of the gut, at 2.5 mm. It soon takes the form
of a thick walled vesicle containing a cavity. Solid cords
of cells grow out from the entodermal sac. The vesicle
from which these cords develop become the gall bladder and
the larger bile ducts, while the cells become solid cords
between which develop.bile capillaries.^
Jejunum and ileum. In the material under examination
it is impossible to determine the exact point at which the
duodenum stops and the jejunum begins. The exact cell
structure is poorly indicated, and the glands, as such, are
not distinguishable. According to Prentiss,^ the intestinal
glands in the human appear as ingrowths of epithelium about
the base of the villi. They develop first at 90 mm. in
the duodenum, and at 130 mm. in the jejunum. The glands of
Brunner appear at the fourth month, according to Brand.^
According to Berry^ glands are present throughout the intes
tine at 130 mm. According to Johnson,^ the villi appear in
the 22 mm. human embryo as rounded elevations of epithelium
at the cephalic end of the jejunum. The villi and val-
vuli conniventes are not apparent in the writer’s 18 mm.
horse, nor are the mucous follicles, crypts of Lieberkuhn,
Peyer’s patches, or solitary glands, all of which, however,
iDodds, G. S. op.cit.
^Prentiss, C. W. op.cit.
sibid. ----
4Ibid.
Sibld.
are found in the adult horse
The jejunum, according to Vaughn,^ in discussing the
adult horse, comprises two-fifths of the small intestine.
It is floating and is situated within the free margin of the
mesentery, as is also the remainder of the small intestine.
The jejunum and the ileum, with the exception of the origin
and the last few feet of the mesenteric portion of the in
testine, vary so much in position that only general state
ments can be made. As to the lumen of the intestine in the
embryo horse, the decrease in size, from duodenum to ileo
cecal valve, is gradual, and there is no distinct demarca
tion between the various parts. In the adult cat, according
to Reighard and Jennings,^ the duodenum, at a distance of
8 to 10 cm. caudad of the pylorus, makes a "U" shaped bend,
and here passes without definite limit into the jejunum.
Lillie,4 says, in speaking of the chick,"The main divisions
of the intestine are marked out by their position, size-
relations, and structure before the closure of the yolk
stalk."
The growth in length of the small intestine of the
18 mm. horse embryo has been more rapid than other parts '
of the digestive tract, and, as recorded for the human by
McMurrich,^ by Minot,® and by Prentiss,'^ it has been thrown
Jsd ssbh;Ü pept imus r' oplbit.
^Vaughn, op.cit.
^Reighard, J. and Jennings, H.S. The Anatomy of the Cat.
Henry Holt and Co., 1901.
^Lillie, F. R. Development of the Chick. H. Holt & Co. N.Y. 1908.
^McMurrich, James Playfair, Development of the Human Body.
P. Blakiston*s Sons and Co., Phila., 1913.
Minot, Charles Sedgewick, Laboratory Textbook of Embryology.
P. Blakiston’s Sons and Co., Phila., 1911.
"Prentiss, C. W. op.cit«
2€
into coils. According to Dodds,^ these coils in the 19
mm. human cannot be contained in the body of the embryo and,
hence, extend into the base of the umbilical cord. Minot,^
in discussing the human embryo, states that, "The body cavity
is prolonged into the cord, forming a central space in which
are lodged the loops of the intestine and the allantois."
This same condition is found in the writer’s 18 mm. embryo
horse (Pig. 8).
Certain changes have taken place in the horse embryo
of this stage, which, according to Prentiss,® have likewise
occurred in the 17 mm. human embryo. These changes con
sist of coilings and torsions, due to rapid elongation of
the intestine; differentiation into specific regions; ro
tation of intestine so that the caudal limb lies to the left
and in front of the cephalic limb. Marshall,^ in discussing
the human embryo, states that development of the intestine
is largely a lengthening of the vitelline loop which gives
rise to the entire small intestine beyond the duodenum. In
the human embryo are six primary loops, the first is the
duodenum and the remaining five are contained within the
coelom of the umbilical cord. These remaining five are
thrown into secondary coils. In the 18 mm. embryo horse,
six primary coils are also found, the first being the duo
denum. The second, hov/ever, is contained, not in the coelom
^Dodds, G. S. opcit.
^Minot, C. S. op.cit.
®Prentiss, C. W. op.cit.
^Marshall, A. M., Vertebrate Embryology, Putnam, London, 1893.
23
of the umbilical cord, but in the body cavity proper. The
right cephalic limb of this loop extends to the left side
of the coelom falling caudad to the left limb which twists
over to the right side of the body of the embryo to extend
out into the cord where it bends to form a third loop (Pig.
7), and this third primary loop is probably comparable to
the vitelline loop of the human, spoken of by Prentiss,^
McMurrich,^ and Minot.®
Yolk Stalk. The yolk stalk in the 18 mm. horse, appears
as a small projection from the convex surface of the bend
of the third loop. It is very short and ends blindly. Ac
cording to Dodds,^ in discussing the human embryo, the yolk
stalk arises from about the middle of the small intestine,
and its lumen closes at the 9 mm. stage, though Prentiss,®
also speaking of the human embryo, says that lumen may per
sist until the 14 ram. stage. The small projection is iden
tified as Meckel’s diverticulum (Pig. 5), and marks the
place at which the stalk was previously connected to the in
testine. In the writer’s embryo the wall of thé diverti
culum measures 100 micra and the lumen 60 mlera.
^Prentiss, C. W., op.cit.
^McMurrich, J. P., op.cit.
®Minot, C. S., op.cit.
^Dodds, G. S., op.cit.
®Prentiss, C. W., op.cit.
2:4
LARGE INTESTINE
Caecum and Vermiform Appendix. On the left limb, in
the region of the boundary between the small and large in
testine, in the 18 mm. horse, there has developed a rela
tively large pouch, the caecum and vermiform appendix, which
marks the anterior limit of the large Intestine (Pig. 8).
The portion of the tube extending from this point to the
cloaca has not elongated so rapidly as the small intestine.
However, it is approximately one-third the length of the en
tire intestine. It presents a bend which is on the left
side of the embryo in the body cavity, into which the large
intestine extends from the exo-coelom of the umbilical cord
(Pig. 8). Longitudinal bands, sacculations, glands, and
cell structure, features of demarcation of the large intes
tine, are not indicated in the cross sections. The wall
is cylindrical and thick, and the lumen is somewhat larger
than in the small intestine.
McMurrich^ describes the development of the caecum,
in the human, as that part which arises as a lateral out
growth from the colon, and which continues to increase in
size until it forms a conical pouch just where it joins the
small intestine. The enlargement of its terminal portion.
»
does not keep pace with that portion nearest the intestine,
but gradually becomes more and more differentiated from it
by its lesser caliber, and gives rise to the vermiform
^McMurrich, J. P., op.cit.
2:5
appendix. These structures in the 18 mm. horse occupy a
position on the left side within the umbilical cord. They
take a position on the right side when drawn into the body
cavity, where they are found in the adult horseThe part
identified as the caecum is relatively large and projects
caudo-dorsalwards. In the adult animal, according to Vaughn,^
the large cul-de-sac commences in the right iliac region,
passes obliquely downwards and forwards, terminating in the
left hypochondriac region in a blind pointed extremity*
In the 18 mm. horse the caecum is approximately .68 mm.long
while in the adult, according to Vaughn,® it is 36 in. long,
has a capacity of 6 gal., and is conical in shape. The sup
erior extremity is called the base, or arch, and presents a
convex surface directed backwards, and a concave one directed
forwards. In the concave curvature the ileum terminates and
the large colon originates. It is fixed to the abdominal
wall, posteriorly, by cellular tissue, and to the origin of
the colon by a fold, the meso-colon. The interior presents a
number of transverse ridges corresponding to the exterior
ones. The two orifices are in the concave curvature. The
inferior one marks the termination of the ileum and is guarded
by the ileo-cecal valve. The superior orifice leads to the
great colon. In the 18 ram. horse the transverse ridges are
not identified but the inferior and superior orifices are
found to be present.
^Sisson, Septimus, Op.cit.
^Vaughn, op.cit.
®Ibld.
26
Great Colon and Floating Colon. There is a region of
large intestine in the 18 mm. animal which corresponds to
the great colon of the adult. In the adult, this portion
is marked by longitudinal bands and transverse furrov/s ; it
is doubled on itself forming two arras of equal length, the
terminal portion is near the anterior extremity, and the
convexity is directed forwards. The embryonic colon differs
from that described for the adult in that it passes for a
short distance obliquely forward from the arch of the caecum,
then curves obliquely forward and upward to the region of
the floating colon (Pig. 8). This part forms a sharp "U"
shaped curve with the great colon and is then directed up
ward and backward to a position midway betv/een the internal
iliac arteries where it becomes the rectum (Pig. 6).
Rectum and Cloaca. In the 18 ram. horse embryo, as in
the adult, the rectum, from its beginning at the entrance
to the pelvis, extends posteriorwards in a median plane to
the anus. Marshall^ explains the course taken by the rec
tum, in the human embryo, by the fact that it grows very
slowly and therefore remains straight. The rectum, according
to Dodds,^ is formed by a division of the cloaca into dor
sal and ventral portions by a transverse partition. The
dorsal division becomes the rectum and the ventral becomes
the uro-genital sinus. This division is complete in the
iMarshall, A. M..op.cit.
^Dodds, G. S., op.cit.
2:7
human hy 15 mm., and is also found to he complete in the 18
mm. horse embryo (Fig. 21). Gaudally, the intestinal tube
expands to form the cloaca. This dilatation of the posterior
part of the hind gut, is evident in the chick, according to
Patten,^ by the fourth day of incubation. In the adult
bird, the cloaca, which persists in this species, is present
as a "common chamber into v/hich the intestinal contents,
urine, and products of the reproductive organs are received
for discharge." In the higher forms, the cloaca divides in
the manner described above. Occasionally, there is a failure
to divide which results an "anomalous relation between the
digestive and uro-genital tracts", says Dodds,^ in speaking
of the human. Thus, the condition found in the normal 16 mm.
human embryo is similar to that in the horse at the 18 mm.
stage.
Allantois. In the 18 mm. horse the allantois, a di
verticulum of the alimentary tract, into which it opens
immediately in front of the anus, arises from the cloaca
and extends into the umbilical cord. Its length at this
stage is 2.4 mm. According to Dodds,® in speaking of the
human, "The proximal portion of the allantoic stalk is the
homologue of the urinary bladder of mammals and opens dir
ectly into the cloaca, and the ducts draining the excretory
organs open into either side of the allantoic stalk."
Patten, Bradley, M., Early Embryology of the Chick. P.
Blakiston’s Sons and Co., Phila., 1927.
SDodds, G. S., pp.cit.
®Ibid. ---
28
In the 18 mm. horse, the allantois, which was not
modeled, was however traced on the slides through the length
of the umbilical cord to the region of the rectum, where it
divided into two parts (Pig. 33), and caudad approaches the
widening urinary bladder (Pig. 34), with ivhich it is con
tinuous (Fig. 35). Two tubles, the ureters, extending from
the metonephritic kidneys (Pig. 37), open into the embryonic
bladder (Fig. 36), and another tubule, the urethra, leads
from the urinary bladder to an outside opening near the
anus (Pig. 37).
The Wolffian ducts, right and left, are present, in
cross section, for only a short distance in the region of
the ureters, and soon become occluded (Pig. 23).
Anal Region. Rupture of the cloacal membrane has
occurred in the embryonic horse of the 18 mm. stage, and the
rectum is in communication with the exterior. The communi
cation formed constitutes the anal opening (Pig. 31). Ac
cording to Patten,1 "The cloaca of the chick shows indica
tion of an opening to the outside on the fourth day of
incubation........A ventral out-pocketing of hind-gut arises
on the caudad to the point at which the allantoic stalk
opens into the cloaca, and at the same time a depression
appears in the overlying ectoderm. The depression external
to the gut pocket is the proctodaeum and the double epithelial
layer formed by the meeting of the gut entoderm and the
proctodaeum ectoderm is the cloacal membrane."
Ipatten, B. M. op.cit.
29
Mesentery. The ventral mesentery is lost at the 18
mm. stage in the horse, while the dorsal persists (Fig. 21)
The jejunum and ileum are suspended in the free margin of
the mesentery proper, but the caecum, great colon, floating
colon, and rectum also have their mesenteries, meso-caecum,
meso-colon, colic mesentery, and meso-rectum respectively.
30
BLOOD VESSELS
Only some of the more important blood vessels will be
considered. The general circulation in the adult horse
(Pig, 4), possesses an anterior aorta, as the main trunk
extending forward from the heart, and is comparable to the
innominate artery of the human. Inthe 18 mm, embryo horse
this blood vessel extends forward through the neck ventral
to the esophagus and trachea (Fig, 7), In the region of the
heart the main arterial trunk takes a position posterior
to the esophagus, and from here on, to its termination, is
known as the dorsal aorta. In the embryo, the dorsal aorta
is no larger than the anterior aorta, but is approximately
twice as long. From the region of the heart it passes at
first upward and then backward (Pig. 6).
The first branch given off is identified as the coeliac
axis, or COGliac-mesenteric artery. This axis consists of
three small arteries branching from the main trunk, extending
independently downward for a short distance (Fig. 6), to form
one large vessel. From this are put off large irregular
sinuses to the liver, and two arteries. The first of the
arteries is identified as the mesenteric, and the other,
which seems to be the extension of the original branch, is
traced down through the umbilical cord between the two
limbs of intestine and is undoubtedly the superior mesenteric
artery (Fig. 6).
In the 18 mm. horse (Fig. 24) the internal iliacs exist
as the hypogastrics or umbilical arteries. Their course.
31
except for a few bends, is down through the cord, one on eithœ
side of the allantois. They maintain an equal size and re
lationship. At their origin they are large and trunk-like,
but, as they are traced to their exit at the body wall, they
become progressively smaller, the right umbilical artery
leaving at a higher level than does the left one.
32
SUMMARY
1. The esophagus of the 18 ram. horse is a hollow tube
4.50 ram. in length, gradually tapering to the stomach. Its
course and relations are approximately the same as those of
the adult horse.
2. The stomach resembles that of the adult in its
general position and in its relationships to the surrounding
organs. As does the adult, it possesses two surfaces, two
boundaries, two extremities, and two orifices. The left
extremity is directed dorsally. And the lining of the entire
organ as revealed in the cross sections shows many folds.
3. The duodenum is similar to that of the adult animal..
The two pancreatic ducts enter the duodenum at separate
levels. The bile duct entersi the intestine with the duct
of the dorsal pancreas. The horse, at this stage, has no
trace of a gall bladder or cystic duct. Polygonal cells
pack the liver, but radiating cords of cells are not indi
cated. Bile and blood capillaries are not present in the
liver tissue at this stage.
4. Intestinal glands, villi, and valvuli conniventes
are not indicated in the 18 mm. stage. Some of the intestine
has herniated into the coelom of the umbilical cord. The
small intestine exhibits six primary bends, or loops, two of
which are contained within the body cavity proper and the
remaining four in the body cord. This condition differs
from that of the human embryo at the same stage in which the
first coil is in the body cavity and the other five in the
33
umbilical cord.
5. The yolk stalk of the 18 mm. horse appears as a small
sacculation, Meckel’s diverticulum, on the third intestinal
bend. Its lumen is 60 micra and its wall 100 micra in
length.
6. The caecum and appendix of the 18 mm. horse are
large. They are contained within the cavity of the umbili
cal cord. The great colon of the embryonic horse follows
a more direct course than does the great colon in the adult.
The floating colon extends from the great colon to the rec
tum. The rectum lies in the median plane of the body of
the embryo.
7. The division of the cloaca into rectum and urogenital
sinus is complete. The base of the allantoic stalk widens
contributing to the formation of the urinary bladder, which
receives two ducts, the right and left ureters. The urethra
extends from the urinary bladder to its exterior opening
near the anus. The Wolffian ducts appear as vestigial
structures, the embryo being a female.
8. Rupture of the cloacal membrane has occurred,
forming the anus.
9. The ventral mesentery of the entire intestine has
disappeared at this stage, while the dorsal persists.
10. The anterior aorta lies ventral to the trachea and
esophagus and is the main arterial trunk supplying the
region anterior to the heart. It is continuous with the
dorsal aorta which lies dorsal to the esophagus and is the
34
main arterial trunk posterior to the heart. Prom the dorsal
aorta arise the coeliac axis and superior mesenteric artery,
the intercostala, renais, and ovarians, and the common iliac
arteries. At the 18 mm, stage the internal iliacs are iden
tified as right and left umbilical arteries extending into
the umbilical cord. The superior mesenteric artery also
extends into the exo-coelom. Its course lies between the
intestinal limbs.
3 5
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Bailey, Frederick Randolph, Textbook of Embryology. W. Wood
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Carnegie Institution of Washington, Contributions to Embryo
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Chauveau. A,, Comparative Anatomy. D, Appleton and Co,,
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Dadd, George H,, Anatomy and Physiology of the Horse. John
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Davison, Alvin, Mammalian Anatomy. P. Blakiston’s Sons and
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Denning, Nettie Ellen, Some Phases in the Embryonic Develop
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Dodds, Gideon S., Essentials of Human Embryology. J. Wiley
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Ewart, James Cossar, The Multiple Origin of Horses and
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Studies in the Development of the Horse.
Trans. Roy. Soc. Edin,, 1915, Vol. 51, Part 2,
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Flower, Frederick B., Evolution of the Horse. Marshall
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Foster, M., The Elements of Embryology. Macmillan and Co., 1898
Gay, Carl Warren, Productive Horse Husbandry. J. B. Lippen-
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Harrison, B, M,, and Denning, N. E., Embryonic Development
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Harrison, B, M.,"A Modification of Reconstruction Methods",
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Description of a New Species of Mastodon.‘
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Kansas. Proceedings of the U. S. National
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Saunder’s and Co., Phila. and London, 1913.
Huber, Cotthelf Carl, Development of the Albino Rat, J. of
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Jenkinson, John Wilfred, Experimental Embryology. Oxford
Clarendon Press, 1909.
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Clarendon Press, 1913.
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Umbilical Cord of Pig, Amer. J.
Anat., XXVI, 1919.
Jordan, Harvey Earnest and Kindred, James Earnest, Embryology.
D. Appleton and Co., N.Y. 1926.
Keibel, Franz, and Mall, Franklin P., Human Embryology.
Vol. II, J. B. Lippincott Co., Phila. and
London, 1912.
Kellicott, William Erkstine, Outline of Chordate Develop
ment, H. Holt and Co., N.Y., 1913.
______ Textbook of General Embryology.
Henry Holt and Co., N.Y., 1913.
Kingsley, J. S., Comparative Anatomy of Vertebrates. P.
Blakiston’s Sons and Co., Phila., 1917.
Lillie, Frank Rottray, Development of the Chick. Henry
Holt and Co., N.Y., 1908.
Loomis, Frederick B., Evolution of the Horse. Marshall
James Co., Boston, 1926.
Lowrey, Lawson G., Prenatal Growth of the Pig, Amer. J. of
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Si?
LycLekker, R ., The Horse and Its Relatives. Macmillan and Co.,
N.Y., 1912.
------ , Some Tibetan Animals. Annual Report Smith. Inst.
1904, Washington, 1905, p. 429-436.
Marshall, A. M., Vertebrate Embryology. Putnam, London, 1893.
Maximow, Alexander A., A Textbook of Embryology. W. B. Saunders
and Co., Phila. and London, 1930.
McEwen, Robert Stanley, Vertebrate Embryology. Henry Holt
and Co., N.Y., 1923.
McMurrich, James Playfair, Development of the Human Body.
P. Blakiston’s Sons and Co., Phila., 1913.
Mentzer, S. H., Anomalous Bile Ducts in Man. J.A.M.A., Oct.
26, 1929, Vol. 93, No. 17, p. 1273.
Minot, Charles Sedgewick, Laboratory Textbook of Embryology.
P. Blakiston’s Sons and Co., Phila., 1911.
Patten, Bradley M., Early Embryology of the Chick. P.
Blakiston’s Sons and Co., Phila., 1927.
_________ Embryology of Pig. P. Blakiston’s Sons
and Co., Phila., 1925.
Prentiss, Charles William, Textbook of Embryology. W. B.
Saunders and Co., Phila., and London, 1915.
Prisse, d’Avenues, E., Egyptian and Arabian Horses. Smith.
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Reese, Albert Moore, The Development of the American Alli
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G. P. Putnam’s Sons, N.Y., 1904.
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38
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39
INDEX TO ILLUSTRATIONS
1. Foetal membranes of the 18 mm. horse. x4.
2. Digestive apparatus of the adult horse - after Megnin,
From Vaughn.
3. Frontal section of stomach and first part of duo
denum of the adult horse. From Sisson.
4. The heart and chief blood vessels of the adult
horse - after Megnin. Prom Vaughn.
5. Right lateral view of the reconstruction of the di
gestive tract and principal blood vessels of the 18 mm.
horse.
6. Left lateral view of the reconstruction of the di
gestive tract and principal blood vessels of the 18 mm. horse.
6a. Left lateral view of the reconstruction of the
digestive tract and principal blood vessels of the 18 mm.
horse, showing method of reconstruction.
7. Left anterior view of the reconstruction of the
digestive tract and principal blood vessels of the 18 mm.
horse•
8. Left posterior view of the reconstruction of the
digestive tract and principal blood vessels of the 18 mm.
horse.
9. Right lateral view of the 18 ram. horse marked to
show the plane of sections. x4.
10. Cross section through pharyngeal region of 18 mm.
embryo, x 70.
11. Cross section through laryngo-tracheal groove of
18 mm. embryo, x 70.
12. Cross section through esophagus and trachea at
level of thyroid anlage. x 22.
13. Cross section showing anterior aorta of 18 mm.
embryo. x 23.33.
14. Cross section through esophagus and trachea at
level of mouth of 18 mm. embryo, x 22.
15. Cross section through esophagus and trachea at
level of thymic lobes of 18 mm. embryo, x 70.
•4 b
16. Cross section through esophagus and trachea at
level of thoracic cavity of the 18 mm. embryo, x 70.
17. Cross section showing bifurcation of trachea of
18 mm. embryo, x 23.33.
18. Cross section through bronchi of 18 mm. embryo.
X 70.
19. Cross section showing umbilical arteries of 18
mm. embryo, x 23.33.
20. Cross section through intestines and lower end of
stomach of 18 mm. embryo, x 23.33.
21. Cross section illustrating rectum, uro-genital
openings, posterior end of coelomic pouches, and bases of
posterior limbs of 18 mm. embryo, x 23.33.
22. Cross section through stomach of 18 mm. embryo, x 70.
23. Cross section illustrating rectum, ureters, and
umbilical arteries of 18 mm. embryo, x 23.33.
24i Cross section showing internal and external iliac
arteries of 18 mm. embryo, x 23.33.
25. Cross section through dorsal aorta and renal ar
teries of 18 mm. embryo, x 23.33.
26. Cross section through pylorus of 18 mm. embryo, x. 85.
27. Cross section showing bile duct, and dorsal and
ventral pancreatic ducts of 18 mm. embryo, x. 85.
28. Cross section through dorsal and ventral pancreatic
ducts of 18 mm. embryo. x. 85.
29. Cross section section through bile and pancreatic
ducts of 18 mm. embryo, x. 85.
30. Cross section through pancreas of 18 mm. embryo, x. 85.
31. Gross section illustrating anal region of 18 mm.
embryo. x. 85.
32. Gross section showing urethra of 18 mm. embryo, x. 85.
33. Cross section through division of allantois of
18 mm. embryo, x. 85.
34. Cross section through allantois and urinary bladder
of 18 mm. embryo, x. 85.
41
35. Cross section showing urinary bladder of 18 mm.
embryo, x 85.
36. Cross section through ureters just previous to
entrance to urinary bladder of 18 mm. embryo, x 85.
37. Cross section showing ureters extending from em
bryonic kidnesy of 18 ram. embryo, x 85.
4(2
INDEX TO ABBREVIATIONS
A A ........... . anterior aorta
A L L ...........allantois
A M ...............amnion
AM G ••••.. amniotic cavity
APP • . • • appendix
AR •••••• anal region
B .......... . bronchus
B B .............bronchi
B D .............bile duct
B L ............. bladder
C .............. caecum
Oil. .......... cleft II
c m cleft III
C A ...............chorion and amnion
G A X ............ coeliec axis
C A ...............coeliac axis
CBD common bile duct
C O ...............coelon
D ............. duodenum
DA ....... dorsal aorta
DP . .........dorsal pancreas
DPD.............dorsal pancreatic duct
E ............... esophagus
EOT.............ectoderm
EIA.............external iliac artery
ENT............. entoderm
4 3
PC ....... . floating colon
G................. glottis
G O ............... great omentum
HC ............... hyoid cartilage
H D ............. .. hepatic duct
HG ............... heart region
I . ............ intestine
IIA ............. internal iliac artery
IL ••••••• • intestinal loop
K . ...........kidney
L ............... liver
LB • ...........lung hud
LP • ...........line of fusion of chorion and allantois
LED. ...........left'hepatic duct
L I ............. . large intestine
LTg............... laryngo-tracheal groove
M ........ . mesentery
MA ............... mesenteric artery
MA.C ............. mandibular cartilage
MD ............. . Meckel’s diverticulum
Mk ............. . Mekel’s cartilage
N •••••••• • notochord
P................. pancreas
P.I............... phryngeal pouch I
P. II • .........pharyngeal pouch II
P.Ill* •••••• pharyngeal pouch III
P C . pleural cavity
PCV............... posterior cardinal vein
44
PH............... pharynx
PLB ....... posterior limb bud
P Y ............. pylorus
R ............... rectum
RA ....... renal artery
RHD . . . . . . . right hepatic duct
S ............... stoAach
SC............... spinalicord
Tc............... thoracic cavity
Th............... thyroid
To............... tongue
TR............... trachea
U ............. . ureter
UA............... umbilical artery
U A L ............. left umbilical-artery
ÜAR ............ right umbilical artery
UB ............. urinary bladder
UO ............ uro-genital opening
UR ....... urethra
V A ............. vitelline artery
VP . ..........ventral pancreas
VPD ............ ventral pancreatic duct
Y f D .............Wolffian duct
Y S .............yolk stalk
4/6
EXPLANATION OP PLATE 2
DIGESTIVE APPARATUS OP THE ADULT HORSE
AFTER MEGNIN .FROM VAUGHN
1. Mouth
2. Pharynx
3. Oesophagus
4. Diaphragm
5. Spleen
6. Stomach
7. Duodenum
8. Liver, upper extremity
9. Great colon
10. Caecum
11. Small intestine
12. Floating colon
13. Rectum
14. Anus
15. Left kidney and ureter
16. Bladder
17. Urethra
a. Hard palate
h. Tongue
c. Soft palate
d. trachea
e. Pulmonary artery
f. Heart
g. Posterior aorta
46
EXPLANATION OP PLATE 4
THE HEART AND CHIEF BLOOD VESSELS OF THE
ADULT HORSE - AFTER MEGNIN. FROM VAUGHN.
1. Heart, right ventricle 21.
2. Heart, left ventricle 22.
3. Heart, left auricle 23.
4. Pulmonary artery 23»
5. Pulmonary veins 24.
6. Anterior aorta 25.
7. Carotid artery 26.
8. Glosso-facial artery 27.
9. Left brachial artery 28.
10. Dorsal artery 28»
11. Superior cervical artery 29.
12. Vertebral artery 30.
12*. Internal thoracic artery 31.
13. Humeral artery 32.
14. Radial artery 33.
14». Cubital artery
15. Great metacarpal artery
16. Ungual artery
17. Posterior aorta
18. Coeliac axis
19. Mesenteric arteries
20. Left renal artery
Small testicular artery
Posterior vena cava
Portal vein
. Hepatic circulation
External iliac artery
Internal iliac artery
Lateral sacral artery
Femoral artery
Posterior tibial artery
. Anterior tibial artery
Metatarsal vessels
Venous plexus of foot
Internal saphenic vein
Cephalic vein
Jugular vein
47
CA
LF
AM
YS,
48
D IG E S T IV E S Y S T E M O F T H E H O R S E
49
Saccus cœciis
{left exlremity)
Œsophagenl region
Œsophagus
1‘gloruH
Fuilrlus
g lm id
rt gion
D ivfrticulum duodcni
Pyloric glond region
50
51
52
CA
RIL
:LIL
53
54
EIA
FC
DA
GC
UAL
M A UAR
AA
PH
55
AA-
MA-^
56
\
9
57
I I
EIII
TO
PH
PII
H.C
PI
MAC
58
59
12
TR
PH
60
IL
61
o
TR
MK
TO.
PH
u
E
62
TM
#
63
mm
TM
,TM
16
TC
64
BB
PC
/.O '
IL
65
DA
BlBl
66
67
SC
PCV
DA
LB
■MI
VA
UAL
-ALL
V B
AH
20
68
DA
LB
GO
UAL
-ALL
0/
BL
69
DA
PC
LB
CO;
22
70
23
71
DA
CAX
Q
M
IIA
EIA
24
72
25
73
L.H.D
. ' RHD
CBD
DPD
VPDJ M A
DPD
VPD
VPD
m
UR
74
LI
-ALL ALL ALL
ALL
CO
UAR
UAL
UB
34 UB
33
UB
35
M
LI
CO
UAR
FC
UAR
UAL
UAL
WD
CO
36
37

Asset Metadata

Creator Wilber, Inez (author)

Core Title The digestive tract of the 18 mm embryo horse

Contributor Digitized by ProQuest (provenance)

Degree Master of Arts

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Biological Sciences,health and environmental sciences,OAI-PMH Harvest

Format application/pdf (imt)

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c39-250926

Unique identifier UC11316601

Identifier EP67061.pdf (filename),usctheses-c39-250926 (legacy record id)

Legacy Identifier EP67061.pdf

Dmrecord 250926

Document Type Thesis

Format application/pdf (imt)

Rights Wilber, Inez

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA