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THE DEVELOPMENT OF THE EXTERNAL FORM OF
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THE GUINEA—PIG (CAVIA COBAYA) BETWEEN
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THE AGES OF ELEVEN DAYS AND TWENTY DAYS
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OF GESTATION‘
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|[[File:Mark_Hill.jpg|50px|left]] This historic 1932 paper by Harman is the first in a series investigating the development of the {{guinea pig}}.
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See also:
{{Ref-Harman1932}}
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{{Ref-Harman1933}}
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MARY T. HARMAN AND MARJORIE PRICKETT’
'''Modern Notes:''' {{guinea pig}}
Kansas State College of Agriculture and Applied Science
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{{Guinea Pig links}}
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{{Historic Disclaimer}}
=The Development of the External Form of the Guinea—Pig (Cavia Cobaya) Between the Ages of Eleven Days and Twenty Days of Gestation=


THREE PLATES (TWENTY-ONE menses)
Mary T. Harman And Marjorie Prickett


CONTENTS
Kansas State College Of Agriculture And Applied Science (1932)
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352


Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
Three Plates (Twenty-One Figures)


Review of literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352


Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
* Contribution no. 136 from the Zoological Laboratory, Kansas State College of Agriculture and Applied Science.
* The authors wish to express their indebtedness to Dr. H. L. Ibsen, of the Animal Husbandry Department, for his cooperation in furnishing the animals for the experiment and his assistance and advice regarding the feeding and care of them.


Breeding conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353


Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 354
==Introduction==


Data and observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . .. 354
Much research has been done on the embryology of the guinea-pig, but there remain many phases which have not been worked out.


Location in the uterus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
==Purpose==


Size of the ova . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 355
It has been our purpose to work out the development of those structures which determine the external form, as well as to supplement the works on the growth in weight and length of embryos of the guinea-pig between the ages of eleven days and twenty days of gestation. As our tables show, we have obtained many embryos of known copulation ages at intervals of every few hours throughout the day, thereby being able to study closely the slight but important changes occurring.


Variations in development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 357
==Review of Literature==


The fetal membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
Earlier investigators, Reichert (’61), Bischoff (352), Hensen (’76), Duval (’92), and Von Spee (’01), contributed chiefly to the knowledge of the development of the very young blastocyst and the formation of the fetal membranes. Recently, Maclaren (’26), Wilson ( ’28), and Hill and Sansom (’29) combined the contributions of these earlier workers with their own investigations and thus brought the development of the blastocyst up to about the tenth day. sDraper (’20) and Ibsen (’28) worked out the growth of the guinea-pig embryos as to weight and length. Ibsen used only those embryos twenty days and older at intervals of five days. Draper observed some few cases of the stages as early as ten days, but neither Draper nor Ibsen studied the structural development. Adloff (’04), Gruber (’06), Loeb (’O6), Weidekowich (’O7), Granzer (’08), Lohle (’13), Rabl (’13), Huber (’18), Dowd (’29), and others have contributed to the information concerning the development of tissues, organs, glands, etc.


Early stages of differentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359


The fourteen-day embryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 360
==Material and Methods==


The fifteen-day embryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364
iAll of the animals used for this research were obtained through the cooperation and courtesy of the Animal Husbandry Department. The majority of the females, which were discardsfrom the genetics research, were secured when about a month old.


The sixteen-day embryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
===Breeding conditions===


The seventeen-day embryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 367
Conditions which might give rise to variations in the development of the embryos were eliminated as nearly as possible by keeping the breeding conditions uniform. The animals were kept in metal cages in the basement of a limestone building, so there was little range of temperature. A balanced ration of rolled—oats mixture, alfalfa hay, either sprouted cats or fresh green plants, and water was regularly provided the animals each day. Since the feeding was uniform and adequate, any differences in degree of development could hardly be interpreted on the basis of nutrition.


The eighteen-day embryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367


The nineteen-day embryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368
With a few exceptions, only young virgin females were used. These animals when secured were weighed, recorded, ear-tagged, and kept in cages separate from the males. At a regular time each day each female showing an open vagina was removed to a cage with a male and closely observed. If copulation occurred, the vagina was examined for the presence of a vaginal plug. In each case the record was made on the female’s card and in the general record book, as, for instance: (C-6 9 X A-1 8 ) 8.00 -A.M.,. December 2, 1929; vaginal plug. Thus a double check was kept which eliminated chances for error. This was desirable because the age of each litter used was in every case the exact copulation age calculated from the time of copulation to the time of killing the female. A


The twenty-day embryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 369


Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
As a result of these careful observations only a few nonpregnant females were sacrificed. A total of ninety-four females was killed. Of the sixty-eight which showed a vaginal plug after copulation, sixty-one were found to be pregnant. Of the four showing no vaginal plug, two were pregnant and two were non-pregnant. Twenty-two cases were uncertain and of that number only nine Were pregnant.


Summary . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
===Procedure===


Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372
Before killing a female for dissection, all supplies were in readiness so that no delays need occur in removing the embryos. The female was again Weighed and then killed. The use of illuminating gas was found to be the most satisfactory Way to kill the animal.


‘Contribution no. 136 from the Zoiilogical Laboratory, Kansas State College
of Agriculture and Applied Science.


“The authors wish to express their indebtedness to Dr. H. L. Ibsen, of the
The entire uterus with its contents was left in place in the animal until a drawing had been made showing the relative size and location of each embryo in the uterus. Embryos in the right uterine horn were indicated as R 1, R 2, R 3, etc., from the union of the horns distally to the fallopian tubes. In a similar manner in the left horn the embryos were accordingly designated as L 1, L 2, etc.
Animal Husbandry Department, for his cooperation in furnishing the animals
for the experiment and his assistance and advice regarding the feeding and care
of them.


351
352 MARY T. HABMAN AND MARJORIE PRICKETT


INTRODUCTION
By cutting the uterine Wall carefully along the antimeso-e metrial side, it was possible to remove each embryonic blastocyst intact along with the deciduate maternal tissue.


Much research has been done on the embryology of the
guinea-pig, but there remain many phases which have not
been worked out.


Purpose
Before further dissecting the blastocyst it was carefully weighed on an analytical balance, sensitive to 0.001 gram. Embryos of sixteen days and older were removed from the blastocyst, Weighed, measured by means of a vernier caliper, and then fixed in Bouin’s fixative solution. Lastly, the remaining fluid and the membranes Were Weighed separately. In the case of embryos younger than fifteen days the total Weight of the blastocyst with the deciduate tissue was so small that it was found to be impractical to Weigh the embryo, fluids, and membranes separately. Some fifteen-day embryos were Weighed apart from the rest of the blastocyst, but the chance for error on such small Weights is as great as the significance of the Weight.


It has been our purpose to work out the development of
==Data and Observations==
those structures which determine the external form, as well
as to supplement the works on the growth in weight and
length of embryos of the guinea-pig between the ages of
eleven days and twenty days of gestation. As our tables
show, we have obtained many embryos of known copulation
ages at intervals of every few hours throughout the day,
thereby being able to study closely the slight but important
changes occurring.


Review of literature
In tables 4 and 5 the majority of the litters obtained are recorded as to the age of the litter, the location of each blastocyst in the uterus, the Weight and length of the embryos, and the Weights of the membranes and fluids.


Earlier investigators, Reichert (’61), Bischoff (352), Hensen (’76), Duval (’92), and Von Spee (’01), contributed
chiefly to the knowledge of the development of the very young
blastocyst and the formation of the fetal membranes. Recently, Maclaren (’26), Wilson ( ’28), and Hill and Sansom
(’29) combined the contributions of these earlier workers with
their own investigations and thus brought the development
of the blastocyst up to about the tenth day. sDraper (’20)
and Ibsen (’28) worked out the growth of the guinea-pig
embryos as to weight and length. Ibsen used only those
embryos twenty days and older at intervals of five days.
Draper observed some few cases of the stages as early as ten
days, but neither Draper nor Ibsen studied the structural
development. Adloff (’04), Gruber (’06), Loeb (’O6), Weidekowich (’O7), Granzer (’08), Lohle (’13), Rabl (’13), Huber
(’18), Dowd (’29), and others have contributed to the information concerning the development of tissues, organs,
glands, etc.


===Location in the uterus===


MATERIAL AND METHODS
A total of 213 embryos was obtained from seventy-one litters. Reference to the table shows that there is a rather uniform distribution in the two horns of the uterus. The combinations most frequently found are ‘two right-one left’ and ‘one right-two left.’


iAll of the animals used for this research were obtained
through the cooperation and courtesy of the Animal Husbandry Department. The majority of the females, which
were discardsfrom the genetics research, were secured when
about a month old.
Breeding conditions
Conditions which might give rise to variations in the development of the embryos were eliminated as nearly as possible by keeping the breeding conditions uniform. The
animals were kept in metal cages in the basement of a limestone building, so there was little range of temperature. A
balanced ration of rolled—oats mixture, alfalfa hay, either
sprouted cats or fresh green plants, and water was regularly
provided the animals each day. Since the feeding was
uniform and adequate, any differences in degree of development could hardly be interpreted on the basis of nutrition.
With a few exceptions, only young virgin females were
used. These animals when secured were weighed, recorded,
ear-tagged, and kept in cages separate from the males. At a
regular time each day each female showing an open vagina
was removed to a cage with a male and closely observed.
If copulation occurred, the vagina was examined for the
presence of a vaginal plug. In each case the record was
made on the female’s card and in the general record book,
as, for instance: (C-6 9 X A-1 8 ) 8.00 -A.M.,. December 2, 1929;
vaginal plug. Thus a double check was kept which eliminated
chances for error. This was desirable because the age of
each litter used was in every case the exact copulation age
calculated from the time of copulation to the time of killing
the female. A
As a result of these careful observations only a few nonpregnant females were sacrificed. A total of ninety-four
females was killed. Of the sixty-eight which showed a
vaginal plug after copulation, sixty-one were found to be
pregnant. Of the four showing no vaginal plug, two were
354 MARY '1‘. HARMAN AND MARJORIE PRICKETT
pregnant and two were non-pregnant. Twenty-two cases
were uncertain and of that number only nine Were pregnant.
Procedure
Before killing a female for dissection, all supplies were in
readiness so that no delays need occur in removing the
embryos. The female was again Weighed and then killed.
The use of illuminating gas was found to be the most satisfactory Way to kill the animal.
The entire uterus with its contents was left in place in
the animal until a drawing had been made showing the relative size and location of each embryo in the uterus. Embryos
in the right uterine horn were indicated as R 1, R 2, R 3, etc.,
from the union of the horns distally to the fallopian tubes.
In a similar manner in the left horn the embryos were accordingly designated as L 1, L 2, etc.
By cutting the uterine Wall carefully along the antimeso-e
metrial side, it was possible to remove each embryonic
blastocyst intact along with the deciduate maternal tissue.
Before further dissecting the blastocyst it was carefully
Weighed on an analytical balance, sensitive to 0.001 gram.
Embryos of sixteen days and older were removed from the
blastocyst, Weighed, measured by means of a vernier caliper,
and then fixed in Bouin’s fixative solution. Lastly, the
remaining fluid and the membranes Were Weighed separately.
In the case of embryos younger than fifteen days the total
Weight of the blastocyst with the deciduate tissue was so
small that it was found to be impractical to Weigh the embryo,
fluids, and membranes separately. Some fifteen-day embryos
were Weighed apart from the rest of the blastocyst, but the
chance for error on such small Weights is as great as the
significance of the Weight.
DATA AND OBSERVATIONS
In tables 4 and 5 the majority of the litters obtained are
recorded as to the age of the litter, the location of each blastocyst in the uterus, the Weight and length of the embryos,
and the Weights of the membranes and fluids.
Location in the uterus
A total of 213 embryos was obtained from seventy-one
litters. Reference to the table shows that there is a rather
uniform distribution in the two horns of the uterus. The
combinations most frequently found are ‘two right-one left’
and ‘one right-two left.’


TABLE 1
TABLE 1


The distribution of the blastocysts within the uterus, showing the number of
The distribution of the blastocysts within the uterus, showing the number of litters in which each combination occurred
litters in which each combination occurred


NUMBER or ' INDIVIDUALS IN nmrvmusns IN
NUMBER or ' INDIVIDUALS IN nmrvmusns IN LITTEB3 morn: HORN LEFT HORN
LITTEB3 morn: HORN LEFT HORN


1 1 0
1 1 0
Line 218: Line 112:
13 1 2
13 1 2


20 2 1
20 2 1 Total, 71 113 100
Total, 71 113 100


Size of the ova
Size of the ova


There is a gradual increase in the weight of the total blastecyst and deciduate maternal tissue. The total weight of each
There is a gradual increase in the weight of the total blastecyst and deciduate maternal tissue. The total weight of each was not taken in every case, for occasionally the membranes were broken, allowing the amnionic fluids to escape. Since the fluids have a high specific gravity, the loss of these would aifect greatly the weight of the total.
was not taken in every case, for occasionally the membranes
 
were broken, allowing the amnionic fluids to escape. Since
 
the fluids have a high specific gravity, the loss of these would
It will be noted from our data that in no instance does the weight of the heaviest ovum at any one age closely approach the weight of the heaviest ovum of the succeeding day. However, the weight of the lightest ovum at any one age may fall within the range of weights of the preceding day. The data in table 2 show that the average Weight of the thirteen-day ovum is less than that of the twelfth day. No conclusions may be drawn from this, for the number of cases recorded for those days was small. On the whole, though, there is an increase in Weight from day to day, so that in the ten-day period which We have studied the weight has increased itself almost ten times.  
aifect greatly the weight of the total.


It will be noted from our data that in no instance does the
weight of the heaviest ovum at any one age closely approach
the weight of the heaviest ovum of the succeeding day. However, the weight of the lightest ovum at any one age may fall
within the range of weights of the preceding day. The data
in table 2 show that the average Weight of the thirteen-day
ovum is less than that of the twelfth day. No conclusions may
be drawn from this, for the number of cases recorded for
those days was small. On the whole, though, there is an
increase in Weight from day to day, so that in the ten-day
period which We have studied the weight has increased itself


almost ten times.
TABLE 2
TABLE 2


The weight of the smallest ovum and the heaviest ovum for each age; also the
The weight of the smallest ovum and the heaviest ovum for each age; also the average weight of all ova secured for each age. These figures include the weight of the deciduate maternal tissue
average weight of all ova secured for each age. These figures
 
include the weight of the deciduate maternal tissue
_ RANGE OF WEIGHTS AGE AVERAGE WEIGHT 'Lightest Heaviest Days Grams Grams Grams 20 1.685 1.460 1.935 19 1.325 1.015 1.625 18 ' 1.017 0.809 1.200 17 0.904 0.695 1.060 16 0.662 0.300 (resorbed?) 0.890 15 0.614 0.340 0.800 14 0.477 0.200 0.735 13 0.375 0.252 0.650 12 0.389 0.340 0.455 11 0.197 0.147 0.250 TABLE 3
 
The weight and length of the embryos for each given age, including the average, the smallest, and the largest of the group for each age
 
WEIGHT IN GRAMS LENGTH IN MILLIMETEES AGE, DAYS ‘ Average Minimum Maximum Average Minimum Maximum 20 0.1385 0.080 0.200 9.1 8.0 10.2 19 0.1183 0.060 I 0.245 8.25 6.9 9.7 18 0.0649 0.014 0.095 6.37 5.7 8.1 17 0.0541 0.025 0.107 5.38 4.2 6.7 16 0.0487 0.010 0.075 4.41 4.0 4.9 15 0.0116 0.005 I 0.015 3.73 1.9 _ 5.4
 
Accompanying this gradual increase in the total Weight of the blastocyst and deciduate membranes combined there is a corresponding increase in the Weight and length of the embryo alone. In table 3 it is to be noted that the average Weight of the embryo alone is 0.0116 gram at fifteen days as contrasted with the average Weight of 0.1385 gram at twenty days. There is a comparatively Wide range between the minimum and maximum weight and length of a given age, but this wide range does not alter the increase in the average.


_ RANGE OF WEIGHTS
AGE AVERAGE WEIGHT 'Lightest Heaviest
Days Grams Grams Grams
20 1.685 1.460 1.935
19 1.325 1.015 1.625
18 ' 1.017 0.809 1.200
17 0.904 0.695 1.060
16 0.662 0.300 (resorbed?) 0.890
15 0.614 0.340 0.800
14 0.477 0.200 0.735
13 0.375 0.252 0.650
12 0.389 0.340 0.455
11 0.197 0.147 0.250
TABLE 3


The weight and length of the embryos for each given age, including the average,
Likewise there is the lengthening of the embryo from 3.73 mm. at fifteen days to 9.1 mm. at twenty days. This, as we shall see later, is not the true increase in length, for the shape of the embryo is undergoing such rapid changes that a true measurement of the length on the same basis can hardly be made.
the smallest, and the largest of the group for each age


WEIGHT IN GRAMS LENGTH IN MILLIMETEES
===Variations in development===
AGE, DAYS
‘ Average Minimum Maximum Average Minimum Maximum
20 0.1385 0.080 0.200 9.1 8.0 10.2
19 0.1183 0.060 I 0.245 8.25 6.9 9.7
18 0.0649 0.014 0.095 6.37 5.7 8.1
17 0.0541 0.025 0.107 5.38 4.2 6.7
16 0.0487 0.010 0.075 4.41 4.0 4.9
15 0.0116 0.005 I 0.015 3.73 1.9 _ 5.4


Accompanying this gradual increase in the total Weight of
It is evident that there is some ,variation in the weight of the total blastocyst or the embryo alone, for any given age. These variations may be due to one or more causes. An actual difference in development may occur for the same copulation age, for it is generally accepted that not always the same period of time elapses between copulation and fertilization- At this early stage an hour’s difference in the fertilization age may result in a great difference in the size as well as the appearance of the embryo. Also, some of the variation undoubtedly can be accounted for by mechanical dififerences, i.e., varying amounts of fluids adhering to the materials weighed or varying amounts of membranes remaining attached to the embryo. Embryos at this stage are not at all firm, so that when the length is taken a slightly different degree of curvature may present itself with each embryo. Thus, small diflerences in lengths may be obtained from embryos which are approximately the same actual length. But in spite of these variations, it is self-evident that there is a continual, more or less gradual, development of the ovum throughout the period.
the blastocyst and deciduate membranes combined there is
a corresponding increase in the Weight and length of the
embryo alone. In table 3 it is to be noted that the average
Weight of the embryo alone is 0.0116 gram at fifteen days as
contrasted with the average Weight of 0.1385 gram at twenty
days. There is a comparatively Wide range between the
minimum and maximum weight and length of a given age,
but this wide range does not alter the increase in the average.


Likewise there is the lengthening of the embryo from 3.73
===The fetal membranes===
mm. at fifteen days to 9.1 mm. at twenty days. This, as we
shall see later, is not the true increase in length, for the shape
of the embryo is undergoing such rapid changes that a true
measurement of the length on the same basis can hardly
be made.


Variations in development
In the youngest ova with the decidua which we have removed from the uterus, the placentaleside presents an appearance diifering little from the free end. In the very early eleven-day ovum there may be present still the blood clot marking the fusion of the decidua capsularis over the implanting ovum. In that case the placental pole may be distinguished as being at the end opposite the blood clot. Usually at the eleven-day stage the blastocyst with its membranes is a slightly oblong vesicle, with the long axis running through the placental pole. Examination of the blastocyst alone shows that it is an elongated closed cylinder (fig. 1) lying in the implantation cavity. At the one end, the placental pole, the thickened ectoplacental trophoblast has formed scattered villi which may be separated easily from the maternal tissue. The incomplete ‘inverted’ yolk sac is continuous with the ectoplacental trophoblast. Within the yolk sac at one end, the obplacental pole, is the ectodermal amnioembryonal mass, which by this time has arranged itself in the form of a hollow sphere of cells one layer thick. That portion of the sphere which is toward the placental pole will give rise to the ectoderm of the amnion; the remainder is destined to give rise to the ectodermal parts of the embryo.


It is evident that there is some ,variation in the weight of
the total blastocyst or the embryo alone, for any given age.
These variations may be due to one or more causes. An
actual difference in development may occur for the same
copulation age, for it is generally accepted that not always
the same period of time elapses between copulation and
fertilization- At this early stage an hour’s difference in the
fertilization age may result in a great difference in the size
as well as the appearance of the embryo. Also, some of the
variation undoubtedly can be accounted for by mechanical
dififerences, i.e., varying amounts of fluids adhering to the
materials weighed or varying amounts of membranes remaining attached to the embryo. Embryos at this stage are not
at all firm, so that when the length is taken a slightly different degree of curvature may present itself with each embryo. Thus, small diflerences in lengths may be obtained
from embryos which are approximately the same actual
length. But in spite of these variations, it is self-evident
that there is a continual, more or less gradual, development
of the ovum throughout the period.


The fetal membranes
By the thirteenth day (fig. 2) the blastocyst can be separated from the decidua only with difiiculty, for the villi have continued to form at the placental pole until the ovum is becoming rather well implanted. The blastocyst is tending to widen and flatten (fig. 3). The amnio-embryonal area at thirteen days is well organized into two portions, the amnion and the ectoderm of the embryonic disc. Toward the end of the thirteenth day the anlage of the allantois appears as a spongy mesodermal mass at the posterior end of the embryonic disc.


In the youngest ova with the decidua which we have removed from the uterus, the placentaleside presents an appearance diifering little from the free end. In the very early
eleven-day ovum there may be present still the blood clot
marking the fusion of the decidua capsularis over the implanting ovum. In that case the placental pole may be distinguished as being at the end opposite the blood clot.
Usually at the eleven-day stage the blastocyst with its membranes is a slightly oblong vesicle, with the long axis running
through the placental pole. Examination of the blastocyst
alone shows that it is an elongated closed cylinder (fig. 1)
lying in the implantation cavity. At the one end, the placental
pole, the thickened ectoplacental trophoblast has formed
scattered villi which may be separated easily from the maternal tissue. The incomplete ‘inverted’ yolk sac is continuous with the ectoplacental trophoblast. Within the yolk sac
at one end, the obplacental pole, is the ectodermal amnioembryonal mass, which by this time has arranged itself in
the form of a hollow sphere of cells one layer thick. That
portion of the sphere which is toward the placental pole will
give rise to the ectoderm of the amnion; the remainder is
destined to give rise to the ectodermal parts of the embryo.


By the thirteenth day (fig. 2) the blastocyst can be separated from the decidua only with difiiculty, for the villi have
The allantois enlarges and becomes more spongy, so that by the fifteenth and sixteenth days it is almost as large as the embryo itself. By the seventeenth or eighteenth day it has assumed a more stalked appearance and later it fuses with the placental area to form the allantoic placenta. Lying close beside the allantoic stalk are the umbilical vein and umbilical arteries carrying blood from and to the placenta. Although the allantois attains such an extended degree of development, it is practically devoid of endoderm. The endo dermal diverticulum is so short that there is little or no endoderm out of the body (fig. 17).
continued to form at the placental pole until the ovum is
becoming rather well implanted. The blastocyst is tending
to widen and flatten (fig. 3). The amnio-embryonal area at
thirteen days is well organized into two portions, the amnion
and the ectoderm of the embryonic disc. Toward the end of
the thirteenth day the anlage of the allantois appears as a
spongy mesodermal mass at the posterior end of the
embryonic disc.


The allantois enlarges and becomes more spongy, so that
by the fifteenth and sixteenth days it is almost as large as
the embryo itself. By the seventeenth or eighteenth day it
has assumed a more stalked appearance and later it fuses
with the placental area to form the allantoic placenta. Lying
close beside the allantoic stalk are the umbilical vein and
umbilical arteries carrying blood from and to the placenta.
Although the allantois attains such an extended degree of
development, it is practically devoid of endoderm. The endo
dermal diverticulum is so short that there is little or no
endoderm out of the body (fig. 17).


The yolk sac of the guinea-pig is peculiar in that it forms not a closed vesicle as in most other mammals, but an ‘inverted’ vesicle, presenting the endodermal surface to the cavity of the uterus, and at the edge distally from the embryonic disc it is fused with the ectoplacental trophoblast (fig. 18). However, it nowhere forms a villous attachment. The circulation of the yolk sac is carried on through the vitelline veins and arteries. Thereis no crossing over of blood vessels from the yolk sac to the placenta and vice versa, for in the full-term fetal membranes the yolk sac may be stripped from the placenta with little or no resulting hemorrhage.


The yolk sac of the guinea-pig is peculiar in that it forms
not a closed vesicle as in most other mammals, but an ‘inverted’ vesicle, presenting the endodermal surface to the
cavity of the uterus, and at the edge distally from the embryonic disc it is fused with the ectoplacental trophoblast
(fig. 18). However, it nowhere forms a villous attachment.
The circulation of the yolk sac is carried on through the
vitelline veins and arteries. Thereis no crossing over of
blood vessels from the yolk sac to the placenta and vice versa,
for in the full-term fetal membranes the yolk sac may be
stripped from the placenta with little or no resulting hemorrhage. ,


A thinning of the decidua capsularis and a thickening of
A thinning of the decidua capsularis and a thickening of the decidua basalis enable one to distinguish quickly and easily the placental area. By the nineteenth or twentieth day the embryonic placenta can be separated from the maternal placenta.
the decidua basalis enable one to distinguish quickly and
easily the placental area. By the nineteenth or twentieth day
the embryonic placenta can be separated from the maternal
placenta.


Thus, in the ten-day period which we have studied all the
fetal membranes have become well established and have
become functional.


Early stages of differentiation
Thus, in the ten-day period which we have studied all the fetal membranes have become well established and have become functional.


As before stated, in the blastocyst of the eleven-day stage
===Early stages of differentiation===
not only the primary germ layers are established, but the
yolk sac and ectoplacental trophoblast are rather well defined
and the ectodermal amnio-embryonal mass has split into a
hollow sphere of cells (fig. 1). A section through the elevenday blastocyst (fig. 18) shows the mesodermal cells pushing
outward along the endoderm of the yolk sac; the mesoderm
already lies in a thin layer between the ectoderm and endoderm in the region of the embryonic disc.


Externally at thirteen days, the amnio-embryonal area
As before stated, in the blastocyst of the eleven-day stage not only the primary germ layers are established, but the yolk sac and ectoplacental trophoblast are rather well defined and the ectodermal amnio-embryonal mass has split into a hollow sphere of cells (fig. 1). A section through the elevenday blastocyst (fig. 18) shows the mesodermal cells pushing outward along the endoderm of the yolk sac; the mesoderm already lies in a thin layer between the ectoderm and endoderm in the region of the embryonic disc.
is seen to consist of the embryonic disc and the amnionic
portion. Early in the thirteenth day (fig. 2) the embryonic
disc appears as a dome-shaped plate of cells consisting of
all three germ layers, of which the endoderm, a part of the
‘inverted’ yolk sac, lies externally. Within the cavity between the embryonic disc and the placental portion is the
amnion lying rather close to the dorsal surface of the
embryonic disc.


During the thirteenth day (fig. 3) a rapid proliferation and
growth dorsalward of mesodermal cells occur at the posterior
end of the embryonic disc marking the anlage of the allantois.


The fourteen-day embryo
Externally at thirteen days, the amnio-embryonal area is seen to consist of the embryonic disc and the amnionic portion. Early in the thirteenth day (fig. 2) the embryonic disc appears as a dome-shaped plate of cells consisting of all three germ layers, of which the endoderm, a part of the ‘inverted’ yolk sac, lies externally. Within the cavity between the embryonic disc and the placental portion is the amnion lying rather close to the dorsal surface of the embryonic disc.


The study of the embryo itself really begins with the
During the thirteenth day (fig. 3) a rapid proliferation and growth dorsalward of mesodermal cells occur at the posterior end of the embryonic disc marking the anlage of the allantois.
embryo of the fourteenth day (fig. 4), for at the fourteenth
day the embryonic disc has thickened and formed definite
limits. Along the median line, in the posterior half of the
embryonic disc, is a narrow strip, the primitive streak. This
appears lighter and thinner than the surrounding area, due to
the presence of the primitive groove in the primitive streak.
Marking the anterior end of the primitive streak is a somewhat thickened area, Hensen’s node. Laterally the germ
layers are thickened, especially the ectoderm. However, this
thickening does not extend quite to the edge of the embryonic
disc; thus the area has a wing-like appearance. Anterior to
Hensen’s node and lying between and ventral to the medullary plates is the prominent club—shaped head process (fig.
19).


Further development during the fourteenth day (fig. 5)
===The fourteen-day embryo===
gives the embryo a difi"erent appearance. The head process
no longer occupies such a prominent position; in fact, it is
not noticeable at all in the whole mount. The primitive streak
has shortened and the primitive groove has almost disappeared. Hensen’s node appears as scarcely more than merely
the end of theprimitive streak. In the anterior and lateral
portions of the disc the ectoderm. is thickening decidedly to
form the broad flat medullary plates.


These medullary plates at a slightly further development
The study of the embryo itself really begins with the embryo of the fourteenth day (fig. 4), for at the fourteenth day the embryonic disc has thickened and formed definite limits. Along the median line, in the posterior half of the embryonic disc, is a narrow strip, the primitive streak. This appears lighter and thinner than the surrounding area, due to the presence of the primitive groove in the primitive streak. Marking the anterior end of the primitive streak is a somewhat thickened area, Hensen’s node. Laterally the germ layers are thickened, especially the ectoderm. However, this thickening does not extend quite to the edge of the embryonic disc; thus the area has a wing-like appearance. Anterior to Hensen’s node and lying between and ventral to the medullary plates is the prominent club—shaped head process (fig. 19).
(fig. 6) increase in length much more than in width, and as
 
they do so the medullary groove is formed between them.
 
The head fold, as a very shallow crescentic groove, is begin
Further development during the fourteenth day (fig. 5) gives the embryo a difi"erent appearance. The head process no longer occupies such a prominent position; in fact, it is not noticeable at all in the whole mount. The primitive streak has shortened and the primitive groove has almost disappeared. Hensen’s node appears as scarcely more than merely the end of theprimitive streak. In the anterior and lateral portions of the disc the ectoderm. is thickening decidedly to form the broad flat medullary plates.
ning to raise the head from the embryonic disc. Ventrally
 
361
 
These medullary plates at a slightly further development (fig. 6) increase in length much more than in width, and as they do so the medullary groove is formed between them. The head fold, as a very shallow crescentic groove, is beginning to raise the head from the embryonic disc. Ventrally 361


TABLE 4
TABLE 4


A tabulation of the length and weight of individual embryos within litters of succeeding ages between fifteen and twenty
A tabulation of the length and weight of individual embryos within litters of succeeding ages between fifteen and twenty days, inclusive, and the location of each embryo within the uterus
days, inclusive, and the location of each embryo within the uterus


TIME ELAPSING : INDIVIDUALS IN LITTER
TIME ELAPSING : INDIVIDUALS IN LITTER
Line 436: Line 187:
BETWEEN _ Right horn ! Left horn
BETWEEN _ Right horn ! Left horn


COPULATION AND
COPULATION AND REMOVAL LITTER I R 3 R 2 R 1 ' L 1 L 2 L 3
REMOVAL LITTER I R 3 R 2 R 1 ' L 1 L 2 L 3


N0.
N0.


Weight I Length Weight Length Weight Length Weight Length Weight Length Weight Length
Weight I Length Weight Length Weight Length Weight Length Weight Length Weight Length 3 of ' of of of of of of of 1 of ' of of of embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo
3 of ' of of of of of of of 1 of ' of of of
embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo


.200 9.9 .150 9.3 ' .185 8.6
.200 9.9 .150 9.3 ' .185 8.6 .135 9.4 .160 10.2 .165 9 .080 8 .100 8.8 .110 9.7 .100 8.3.
.135 9.4 .160 10.2 .165 9
.080 8
.100 8.8 .110 9.7 .100 8.3.


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.100 7.3 ' .060 7.6 .110 .110 9.1 .245 9.7 .155 9 .115 9.1 .110 8.6 .130 .115
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TABLE 5
TABLE 5


A tabulation of the weight of the blastoeyst with the adhering maternal tissue and the weight of the same after the embryo
A tabulation of the weight of the blastoeyst with the adhering maternal tissue and the weight of the same after the embryo and the amnionie fluid have been removed. These are the fetal membranes and the adhering maternal tissue of the embryos listed in table 4 and additional blastoeysts of eleven to fourteen days, inclusive
and the amnionie fluid have been removed. These are the fetal membranes and the adhering maternal tissue
of the embryos listed in table 4 and additional blastoeysts of eleven to fourteen days, inclusive


TIME ELAPSING INDIVIDUALS IN LITTER
TIME ELAPSING INDIVIDUALS IN LITTER


BETWEEN .
BETWEEN . COPULATION AND Rlght horn . Left horn
COPULATION AND Rlght horn . Left horn


REMOVAL R3 R2 R1 L1 L2 L3
REMOVAL R3 R2 R1 L1 L2 L3


LITTER
LITTER N0.
N0.


Days Hours
Days Hours


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363
THE AMERICAN JOURNAL OF ANATOMY, VOL. 49, N0. 3


19-24
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this groove produces an internal fold in the endoderm as well as the other germ layers; this internal fold of the endoderm is the first appearance of the foregut. Medially throughout most of the length of the embryonic disc a longitudinal split occurs in the heretofore continuous plate of mesoderm. The segmental plates have been formed by a proliferation of the mesodermal cells at either side of this median slit. From these plates then in turn the first pair of mesodermic somites arises a little posterior to midway in the embryonic disc. The number of somites increases to as high as six pairs by the end of the fourteenth day (fig. 7). Lateral to the meso—~ dermic somites are the lateral plates. At the edge of the head fold the vitelline veins are forming in the splanchnic mesoderm. The head fold is deepening and extending farther posteriorly and in so doing it allows the head, consisting chiefly of the medullary plates and foregut, to curl up dorsally into the amnionic cavity. The medullary plates have continued to thicken, especially anteriorly, and are beginning to be elevated as a pair of folds in the region just anterior to the first mesodermic somite.
===The fifteen-day embryo===


Early in the fifteenth day (fig. 8) probably the most striking thing about the embryo is the broad flat medullary plates, which are so greatly thickened. Anterior to the region of the vitelline veins (fig. 20) the plates are so fiat that there is hardly an indication of the folding except at the most anterior ‘ear~like’ parts, which are really the extreme edges of the plates beginning to fold. From the region of the vitelline veins and proceeding posteriorly (fig. 21) there is a deep groove between the medullary folds, especially in the anterior somite region. However, the plates have not yet approached each other closely enough to fuse.


this groove produces an internal fold in the endoderm as well
as the other germ layers; this internal fold of the endoderm
is the first appearance of the foregut. Medially throughout
most of the length of the embryonic disc a longitudinal split
occurs in the heretofore continuous plate of mesoderm. The
segmental plates have been formed by a proliferation of the
mesodermal cells at either side of this median slit. From
these plates then in turn the first pair of mesodermic somites
arises a little posterior to midway in the embryonic disc.
The number of somites increases to as high as six pairs by
the end of the fourteenth day (fig. 7). Lateral to the meso—~
dermic somites are the lateral plates. At the edge of the head
fold the vitelline veins are forming in the splanchnic mesoderm. The head fold is deepening and extending farther
posteriorly and in so doing it allows the head, consisting
chiefly of the medullary plates and foregut, to curl up dorsally
into the amnionic cavity. The medullary plates have continued to thicken, especially anteriorly, and are beginning to
be elevated as a pair of folds in the region just anterior to
the first mesodermic somite.


The fifteen-day embryo
The folding and differentiation of the splanchnic mesoderm, which had resulted in the vitelline veins at the fourteenth day, have continued and by the fifteenth day have given rise to the heart. At the beginning of the fifteenth day the two bulb-like myocardial foldsxare approaching each other preparatory to fusing into a single heart tube.


Early in the fifteenth day (fig. 8) probably the most striking thing about the embryo is the broad flat medullary plates,
which are so greatly thickened. Anterior to the region of the
vitelline veins (fig. 20) the plates are so fiat that there is
hardly an indication of the folding except at the most anterior
‘ear~like’ parts, which are really the extreme edges of the
plates beginning to fold. From the region of the vitelline
veins and proceeding posteriorly (fig. 21) there is a deep
groove between the medullary folds, especially in the anterior
somite region. However, the plates have not yet approached
each other closely enough to fuse.


The folding and differentiation of the splanchnic mesoderm,
These folds fuse a little later (fig. 20) to form the single heart tube, which grows so rapidly that it begins to twist almost immediately. Simultaneously the folding of the medullary plates has been extended forward. As the folds have become farther extended theyphave also begun to fuse together, forming the medullary or neural tube in the region of the most anterior somites. The medullary folds terminate very abruptly at the anterior end, indicating a late closure of the anterior neuropore. The lateral plates have become much more distinct and show the location of the lateral folds. The number of somites continues to increase, but a difference of two or three somites does not in itself effect much change in appearance now.
which had resulted in the vitelline veins at the fourteenth
day, have continued and by the fifteenth day have given rise
to the heart. At the beginning of the fifteenth day the two bulb-like myocardial foldsxare approaching each other preparatory to fusing into a single heart tube.


These folds fuse a little later (fig. 20) to form the single
heart tube, which grows so rapidly that it begins to twist
almost immediately. Simultaneously the folding of the medullary plates has been extended forward. As the folds have
become farther extended theyphave also begun to fuse together, forming the medullary or neural tube in the region
of the most anterior somites. The medullary folds terminate
very abruptly at the anterior end, indicating a late closure
of the anterior neuropore. The lateral plates have become
much more distinct and show the location of the lateral folds.
The number of somites continues to increase, but a difference
of two or three somites does not in itself effect much change
in appearance now. .


The fusing together of the medullary plates proceeds anteriorly and posteriorly, converting the folds and groove into
The fusing together of the medullary plates proceeds anteriorly and posteriorly, converting the folds and groove into the medullary tube except for the extreme anterior end and the portion in the region of, and posterior to, the last two or three somites (fig. 9). Meanwhile, the walls of the neural tube have been growing more rapidly in some areas than in others, giving the neural tube an irregular bulging form. These bulges may be identified as the primary vesicles of the brain. The optic vesicles are formed as lateral evaginations of the walls of the anterior vesicle. This occurs some time before the medullary plates have fused dorsally.
the medullary tube except for the extreme anterior end and
the portion in the region of, and posterior to, the last two
or three somites (fig. 9). Meanwhile, the walls of the neural
tube have been growing more rapidly in some areas than in
others, giving the neural tube an irregular bulging form.
These bulges may be identified as the primary vesicles of the
brain. The optic vesicles are formed as lateral evaginations
of the walls of the anterior vesicle. This occurs some time
before the medullary plates have fused dorsally.


The heart tube. in the late fifteen-day development has
twisted to the right about as much as in the thirty-six-hour
chick embryo. ‘When the heart first appeared, it was in the
midbrain region, but now it has descended to a position ventral to the hindbrain. The first pair of branchial clefts, although not prominent, is distinguishable. By a continued
folding together of the ventral body wall, the anterior part
of the embryo is raised from the yolk. sac.


The heart tube. in the late fifteen-day development has twisted to the right about as much as in the thirty-six-hour chick embryo. ‘When the heart first appeared, it was in the midbrain region, but now it has descended to a position ventral to the hindbrain. The first pair of branchial clefts, although not prominent, is distinguishable. By a continued folding together of the ventral body wall, the anterior part of the embryo is raised from the yolk. sac.


The sixteen-day embryo


The sixteenth day brings about the complete closure of the
===The sixteen-day embryo===
body Wall ventrally, except for the region of the umbilicus
(fig. 12). This separation of the embryo from the extraembryonic membranes has been accomplished by the pinching
together of the head fold, the lateral limiting sulci, and the
tail fold. In the splanchnopleure the folds unite to form the
alimentary canal, which is now in contact With the yolk sac
only through the yolk stalk. o


The ventralward bending of the head commencing late in
The sixteenth day brings about the complete closure of the body Wall ventrally, except for the region of the umbilicus (fig. 12). This separation of the embryo from the extraembryonic membranes has been accomplished by the pinching together of the head fold, the lateral limiting sulci, and the tail fold. In the splanchnopleure the folds unite to form the alimentary canal, which is now in contact With the yolk sac only through the yolk stalk.
the fifteenth day has increased to form the cephalic flexure.
Due to the closing of the body wall, the embryo tends to lie
on one side and assumes a curvature throughout its length.


The flexure and curvature of the neural tube correspond
to those of the body already described. Continued difier—
entiation of the Walls of the neural tube leads to the more
definite establishment of the primary vesicles of the brain.
The outer Wall of the optic vesicle which had begun as a lateral
evagination from the prosencephalon late in the fifteenth day
invaginates to form the double-Walled secondary vesicle, the
optic cup. Ventrally the Wall of the cup is incomplete, leaving
the opening known as the choroid fissure. An invagination of
the thickened ectoderm external to the optic cup forms a
thick-Walled pit, the lens vesicle. The auditory vesicle, the
otocyst, appears as a pair of thickenings and invaginations of
the ectoderm on the dorsolateral sides of the head a little
anterior to the first somite. These pits deepen and the lips
of the pits begin to fold together at the twenty-three—somite
stage. At about this time the ectoderm in front of the
optic vesicles begins to invaginate to form a pair of pits, the


olfactory pits.
The ventralward bending of the head commencing late in the fifteenth day has increased to form the cephalic flexure. Due to the closing of the body wall, the embryo tends to lie on one side and assumes a curvature throughout its length.
Three or four branchial clefts may be present, but there


is little indication of the maxillary and mandibular processes,
hence the oral fossa is large and open.


Further increase in the number of somites, now ranging
The flexure and curvature of the neural tube correspond to those of the body already described. Continued difier— entiation of the Walls of the neural tube leads to the more definite establishment of the primary vesicles of the brain. The outer Wall of the optic vesicle which had begun as a lateral evagination from the prosencephalon late in the fifteenth day invaginates to form the double-Walled secondary vesicle, the optic cup. Ventrally the Wall of the cup is incomplete, leaving the opening known as the choroid fissure. An invagination of the thickened ectoderm external to the optic cup forms a thick-Walled pit, the lens vesicle. The auditory vesicle, the otocyst, appears as a pair of thickenings and invaginations of the ectoderm on the dorsolateral sides of the head a little anterior to the first somite. These pits deepen and the lips of the pits begin to fold together at the twenty-three—somite stage. At about this time the ectoderm in front of the optic vesicles begins to invaginate to form a pair of pits, the olfactory pits. Three or four branchial clefts may be present, but there is little indication of the maxillary and mandibular processes, hence the oral fossa is large and open.
from sixteen to twenty-three pairs, decreases the length of
the segmental plates.




The seventeen-day embryo
Further increase in the number of somites, now ranging from sixteen to twenty-three pairs, decreases the length of the segmental plates.


The somites now range in number from twenty-three pairs
===The seventeen-day embryo===
to twenty-nine pairs, and the segmental plates have shortened
more (fig. 13). Four branchial clefts may still be present,
but the third and fourth are no longer outstanding. “In many
seventeen-day embryos the fourth pair of clefts has been
closed by the fusing together of the fourth and fifth pairs
of arches. The first pair of arches begins to thicken, especially along the anterior border where the maxillary and
mandibular processes are appearing. These processes are
the primordia of the upper and lower jaws.


The cephalic flexure differs in appearance from the flexure
The somites now range in number from twenty-three pairs to twenty-nine pairs, and the segmental plates have shortened more (fig. 13). Four branchial clefts may still be present, but the third and fourth are no longer outstanding. In many seventeen-day embryos the fourth pair of clefts has been closed by the fusing together of the fourth and fifth pairs of arches. The first pair of arches begins to thicken, especially along the anterior border where the maxillary and mandibular processes are appearing. These processes are the primordia of the upper and lower jaws.
at sixteen days only in that the angle is a little more acute.
The general curvature of the body has increased slightly.
The only change in the neural tube is the further thinning of
the roof plate of the rhombencephalon. The complete separation of the otocyst from the outside ectoderm is effected during the seventeenth day.


The eighteen-day embryo


The curvature of the embryo is becoming more marked in
The cephalic flexure differs in appearance from the flexure at sixteen days only in that the angle is a little more acute. The general curvature of the body has increased slightly. The only change in the neural tube is the further thinning of the roof plate of the rhombencephalon. The complete separation of the otocyst from the outside ectoderm is effected during the seventeenth day.
the region of the first somites and in the region immediately
anterior to the tail (fig. 14). These regions of increased
curvature are the locations of the cervical or nape flexure
and the sacral fiexure. It is not until well toward the end of
the eighteenth day that the sacral and nape flexures become
well established. o p


In addition to the flexures of the neural tube which are
===The eighteen-day embryo===
induced by the body flexures, there is the pontine flexure,
which occurs in the neural tube alone. This third brain
flexure makes its appearance as a ventral bending of the floor
of the anterior portion of the hindbrain or rhombencephalon.
Continued difierentiation of the neural tube initiates the development of the secondary vesicles. A forward and dorsalward expansion of the anterior end of the forebrain gives
rise to the telencephalon, of which the most prominent parts
are the primordia of the cerebral hemispheres. The remainder of the prosencephalon is the diencephalon. Very
little change has occurred in the mesencephalon. Hardly any
expansion occurs in the region between the hindbrain and
the midbrain; this region is now known as the isthmus. Two
divisions from the rhombencephalon arise, the metencephalon
and the myelencephalon. The latter is characterized by the
extremely thin roof.


The eye is assuming a more rounded appearance; the lens
The curvature of the embryo is becoming more marked in the region of the first somites and in the region immediately anterior to the tail (fig. 14). These regions of increased curvature are the locations of the cervical or nape flexure and the sacral fiexure. It is not until well toward the end of the eighteenth day that the sacral and nape flexures become well established.
vesicle is separating from the outside ectoderm and is becoming a spherical body; the choroid fissure, although still present at the beginning of the eighteenth day, has almost disappeared by the end of that day. i


Complete closure of the mouth of the otocyst converts the
vesicle into a closed sac. A conical elevation of the external
side of the most dorsal portion of the otocyst gives rise to
the endolymphatic duct. .


The anterior limb buds arise: as a result of the rapid proliferation of the mesenchymal cells in the region of the sixth
In addition to the flexures of the neural tube which are induced by the body flexures, there is the pontine flexure, which occurs in the neural tube alone. This third brain flexure makes its appearance as a ventral bending of the floor of the anterior portion of the hindbrain or rhombencephalon. Continued difierentiation of the neural tube initiates the development of the secondary vesicles. A forward and dorsalward expansion of the anterior end of the forebrain gives rise to the telencephalon, of which the most prominent parts are the primordia of the cerebral hemispheres. The remainder of the prosencephalon is the diencephalon. Very little change has occurred in the mesencephalon. Hardly any expansion occurs in the region between the hindbrain and the midbrain; this region is now known as the isthmus. Two divisions from the rhombencephalon arise, the metencephalon and the myelencephalon. The latter is characterized by the extremely thin roof.
to eleventh somites. Beginning proliferations of the mesenchyme and a slight swelling posteriorly indicate the hind limb
buds, but as yet they are no more then slight ridges.


The nineteewday embryo


The sacral and nape flexures which had become well defined
The eye is assuming a more rounded appearance; the lens vesicle is separating from the outside ectoderm and is becoming a spherical body; the choroid fissure, although still present at the beginning of the eighteenth day, has almost disappeared by the end of that day.
by the end of the eighteenth day have reached their greatest
development at nineteen days (fig. 15). No new structures
in direct association with the neural tube originate during the
nineteenth day. The optic vesicle is rapidly becoming much
smaller in comparison to the size of the forebrain. The
choroid fissure is no longer visible. Pigment is appearing in
the iris and retinal layer of the vesicle.


The heart, last referred to in the fifteen-day embryo, has i


migrated posteriorly to lie in the body cavity in the region
Complete closure of the mouth of the otocyst converts the vesicle into a closed sac. A conical elevation of the external side of the most dorsal portion of the otocyst gives rise to the endolymphatic duct.
of the fore limb buds. Rapid elongation of the heart tube
between the anterior and posterior fixed ends has produced
a folding. During the nineteenth and twentieth days some
portions of the tube expand more than others and thus there are established the divisions of the heart, of which only the
atrial and ventricular portions are discernibleexternally. The
ventricular portion lies somewhat ventral and posterior to
the atrial portion.


The anterior limb buds, which were mere swellings in the
eighteen-day embryo, have become well-rounded outgrowths.
The hind limb buds are located in the region of the twentyeighth to thirty-third somites.


At the nineteenth day the last somites have been formed;
The anterior limb buds arise: as a result of the rapid proliferation of the mesenchymal cells in the region of the sixth to eleventh somites. Beginning proliferations of the mesenchyme and a slight swelling posteriorly indicate the hind limb buds, but as yet they are no more then slight ridges.
the final number is forty—one pairs. From each anterior
somite there have arisen the sclerotome and myotome, even
before the last few somites have been formed.


The twentyday embryo
===The nineteen-day embryo===


Except for the few most posterior ones, all the somites have
The sacral and nape flexures which had become well defined by the end of the eighteenth day have reached their greatest development at nineteen days (fig. 15). No new structures in direct association with the neural tube originate during the nineteenth day. The optic vesicle is rapidly becoming much smaller in comparison to the size of the forebrain. The choroid fissure is no longer visible. Pigment is appearing in the iris and retinal layer of the vesicle.
differentiated into the myotome, or a muscle plate, and the
sclerotome, which will give rise to the vertebrae (fig. 16).
Development of the neural tube during the twentieth day
produces no new structures, but rather there is a further
definition of the secondary vesicles already established. The
eye, which is becoming relatively smaller in comparison to
the size of the forebrain, shows much pigmentation. The
limb buds are beginning to elongate to the extent that the
distal end is free from the surrounding tissue. The heart
presents an appearance similar to that at nineteen days.


Two pairs of branchial clefts remain unclosed. The maxillary and mandibular processes from the first pair of arches
are enlarging and are growing ventralward, but as yet they
have not united so as to form the face. Hence, the oral fossa
is still wide and open. The olfactory pits are still rather
widely separated.


During the period from the sixteenth day to the twentieth
The heart, last referred to in the fifteen-day embryo, has migrated posteriorly to lie in the body cavity in the region of the fore limb buds. Rapid elongation of the heart tube between the anterior and posterior fixed ends has produced a folding. During the nineteenth and twentieth days some portions of the tube expand more than others and thus there are established the divisions of the heart, of which only the atrial and ventricular portions are discernibleexternally. The ventricular portion lies somewhat ventral and posterior to the atrial portion.
day the umbilical cord has been constricting until by the
twentieth day its connection with the body covers only a
small area.




DISCUSSION
The anterior limb buds, which were mere swellings in the eighteen-day embryo, have become well-rounded outgrowths. The hind limb buds are located in the region of the twentyeighth to thirty-third somites.


The foregoing data as to weight and length parallel those
by Draper (’20). However, Draper took his data from a
fewer number of cases and he recorded embryo blastocysts
of only every second or third day. He gives the weight of
three ova at eleven days as 0.425 gram (average, 0.142 gram
each) as contrasted with our figure of 0.197 gram. Likewise, the weight he records at twenty days is not as heavy as
we have found. On the other hand, his weights given for
fourteen, fifteen, and seventeen days are heavier than we have
secured. But since the weights dealt with were so small and
since the cases recorded were so few, the difference between
our data and those of Draper is of no significance. Moreover, Draper records only a very few cases falling within
the range of ages of the embryos we have observed as to
weight and length, so that a comparison with his results can
hardly be made. Ibsen (’28) records data from embryos only
twenty days of age and older, so no comparison can be made
between these data and his.


The establishing of the germ layers and the development
At the nineteenth day the last somites have been formed; the final number is forty—one pairs. From each anterior somite there have arisen the sclerotome and myotome, even before the last few somites have been formed.
of the blastocyst up to about the tenth day have been worked
out by earlier investigators. Recent investigations and interpretations by Maclaren (’26), Wilson (’28), and Hill and
Sansom (’29) on the earlier phases of development, in addition to the contribution by Duval (’92) covering the fetal
membranes, correspond to and substantiate our investigations
and conclusions. .


Reference to plates 1, 2, and 3 shows that in the ten-day
===The twenty-day embryo===
period studied the embryonic tissue has developed from the
blastocyst showing only the germ layers to the embryo closely
approaching the degree of development of a fetus. Development of the structures such as the eye, ear, limb buds, and
heart is similar to the development of those same structures
in other mammals. On the other hand, the presence of the
‘inverted’ yolk sac, the extensive development of the allantois
almost devoid of endoderm, and the extremely flat medullary
plates anteriorly up to the fifteenth day are quite characteristic of the guinea-pig. The development of these structures
in the other rodents is similar, but does not parallel exactly
the development in the guinea-pig.


SUMMARY
Except for the few most posterior ones, all the somites have differentiated into the myotome, or a muscle plate, and the sclerotome, which will give rise to the vertebrae (fig. 16). Development of the neural tube during the twentieth day produces no new structures, but rather there is a further definition of the secondary vesicles already established. The eye, which is becoming relatively smaller in comparison to the size of the forebrain, shows much pigmentation. The limb buds are beginning to elongate to the extent that the distal end is free from the surrounding tissue. The heart presents an appearance similar to that at nineteen days.


1. The guinea-pig blastocyst presents the ‘inversion’ of the
germ layers.


2. The allantois arises on the thirteenth day as a mesodermal outgrowth, and by the eighteenth or nineteenth day
Two pairs of branchial clefts remain unclosed. The maxillary and mandibular processes from the first pair of arches are enlarging and are growing ventralward, but as yet they have not united so as to form the face. Hence, the oral fossa is still wide and open. The olfactory pits are still rather widely separated.
it fuses with the ectoplacental trophoblast to form the
allantoic placenta.


3. The primitive streak appears late in the twelfth or early
in the thirteenth day and has completely differentiated by the
end of the fourteenth day.


4. The medullary plates arise on the fourteenth day as
During the period from the sixteenth day to the twentieth day the umbilical cord has been constricting until by the twentieth day its connection with the body covers only a small area.
broad flat plates; the plates fuse together first in the hindbrain region on the fifteenth day; the neural tube differentiates into the primary vesicles late on the fifteenth day, and
difierentiates into the secondary vesicles on the eighteenth
day.


5. The first branchial arch is formed at fifteen days, at
==Discussion==
sixteen days the maximum number is present, and at twenty
days three arches are still present.


6. The otocyst appears at sixteen days, closes off at seventeen days, and shows the endolymphatic duct at eighteen days.
The foregoing data as to weight and length parallel those by Draper (’20). However, Draper took his data from a fewer number of cases and he recorded embryo blastocysts of only every second or third day. He gives the weight of three ova at eleven days as 0.425 gram (average, 0.142 gram each) as contrasted with our figure of 0.197 gram. Likewise, the weight he records at twenty days is not as heavy as we have found. On the other hand, his weights given for fourteen, fifteen, and seventeen days are heavier than we have secured. But since the weights dealt with were so small and since the cases recorded were so few, the difference between our data and those of Draper is of no significance. Moreover, Draper records only a very few cases falling within the range of ages of the embryos we have observed as to weight and length, so that a comparison with his results can hardly be made. Ibsen (’28) records data from embryos only twenty days of age and older, so no comparison can be made between these data and his.


7. The optic vesicle evaginates at fifteen days; the choroid
fissure disappears at eighteen days; pigmentation begins at
nineteen days. . t


8. The first pair of somites is formed at fourteen days and
The establishing of the germ layers and the development of the blastocyst up to about the tenth day have been worked out by earlier investigators. Recent investigations and interpretations by Maclaren (’26), Wilson (’28), and Hill and Sansom (’29) on the earlier phases of development, in addition to the contribution by Duval (’92) covering the fetal membranes, correspond to and substantiate our investigations and conclusions.
the final number of forty-one pairs is formed at nineteen days.


9. The body folds off from the yolk sac, except for the
umbilical cord, at sixteen days.


10. The limb buds arise at eighteen days.
Reference to plates 1, 2, and 3 shows that in the ten-day period studied the embryonic tissue has developed from the blastocyst showing only the germ layers to the embryo closely approaching the degree of development of a fetus. Development of the structures such as the eye, ear, limb buds, and heart is similar to the development of those same structures in other mammals. On the other hand, the presence of the ‘inverted’ yolk sac, the extensive development of the allantois almost devoid of endoderm, and the extremely flat medullary plates anteriorly up to the fifteenth day are quite characteristic of the guinea-pig. The development of these structures in the other rodents is similar, but does not parallel exactly the development in the guinea-pig.


11. The cephalic flexure is found at fifteen days and the
==Summary==
sacral and nape flexures at eighteen days.


12. The weight increases from 0.0116 gram at fifteen days
# The guinea-pig blastocyst presents the ‘inversion’ of the germ layers.
to 0.1385 gram at twenty days; the length increases from 3.73
# The allantois arises on the thirteenth day as a mesodermal outgrowth, and by the eighteenth or nineteenth day it fuses with the ectoplacental trophoblast to form the allantoic placenta.
mm. at fifteen days to 9.1 mm. at twenty days.
# The primitive streak appears late in the twelfth or early in the thirteenth day and has completely differentiated by the end of the fourteenth day.
# The medullary plates arise on the fourteenth day as broad flat plates; the plates fuse together first in the hindbrain region on the fifteenth day; the neural tube differentiates into the primary vesicles late on the fifteenth day, and difierentiates into the secondary vesicles on the eighteenth day.
# The first branchial arch is formed at fifteen days, at sixteen days the maximum number is present, and at twenty days three arches are still present.
# The otocyst appears at sixteen days, closes off at seventeen days, and shows the endolymphatic duct at eighteen days.
# The optic vesicle evaginates at fifteen days; the choroid fissure disappears at eighteen days; pigmentation begins at nineteen days.
# The first pair of somites is formed at fourteen days and the final number of forty-one pairs is formed at nineteen days.
# The body folds off from the yolk sac, except for the umbilical cord, at sixteen days.
# The limb buds arise at eighteen days.
# The cephalic flexure is found at fifteen days and the sacral and nape flexures at eighteen days.
# The weight increases from 0.0116 gram at fifteen days to 0.1385 gram at twenty days; the length increases from 3.73 mm. at fifteen days to 9.1 mm. at twenty days.




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WIDAKOWICH, V. 1907 Ueber Entwicklungsdifierenzen des Zentralnervensysterns drier gleichaltiger Embryonen V011 Cavia eobaya. Arb. Ne-urol. Inst. Wien, Bd. 16, S. 452-468.  
Srocsann, O. R., AND G. PAPANICOLAOU 1919 The vaginal closure membrane,
 
copulation, and the vaginal plug in the guinea-pig, with further considerations of the oestrous rhythm. Biol. Bull., vol. 37, pp. 222-245.
WILSON, J. T. 1928 On the question of the interpretation of the structural features of the early blastocyst of the guinea—pig. J our. Anat., vol. 62, pp. 346—358.  
WIDAKOWICH, V. 1907 Ueber Entwicklungsdifierenzen des Zentralnervensysterns drier gleichaltiger Embryonen V011 Cavia eobaya. Arb. Ne-urol.
 
Inst. Wien, Bd. 16, S. 452-468.
 
WILSON, J. T. 1928 On the question of the interpretation of the structural
==Explanation of Plates==
features of the early blastocyst of the guinea—pig. J our. Anat., vol.
 
62, pp. 346—358.
The outlines for the drawings were made with the aid of a microprojector apparatus, so the proportions of the drawings are an exact reproduction of the proportions of the embryos drawn. The figures of plate 3 are photomicrographs.
EXPLANATION OF PLATES


The outlines for the drawings were made with the aid of a microprojector
===Plate 1===
apparatus, so the proportions of the drawings are an exact reproduction of the
proportions of the embryos drawn. The figures of plate 3 are photomicrographs.


PLATE 1
EXPLANATION or fiGURES
EXPLANATION or fiGURES


1 The eleven-day blastocyst. X 10. E, ectodermal amnio—embryona1 mass;
1 The eleven-day blastocyst. X 10. E, ectodermal amnio—embryona1 mass; Y, yolk sac; V, villi; T, ectoplacental trophoblast.
Y, yolk sac; V, villi; T, ectoplacental trophoblast.


2 Early thirteen-day blastocyst. )( 10. D, embryonic disc; A, amnion.
2 Early thirteen-day blastocyst. )( 10. D, embryonic disc; A, amnion.
Line 1,259: Line 623:
3 Late thirteen—day blastocyst. )( 10. L, allantois; B, extra-embryonic blood
3 Late thirteen—day blastocyst. )( 10. L, allantois; B, extra-embryonic blood


vessels.
vessels. 4 Early fourteen-day embryonic disc with the adjacent yolk sac. X 10.
4 Early fourteen-day embryonic disc with the adjacent yolk sac. X 10.


H, head process; N, Hensen’s node; P, primitive streak.
H, head process; N, Hensen’s node; P, primitive streak.
Line 1,268: Line 631:
)< 15. M, medullary plate.
)< 15. M, medullary plate.


6 Ventral view of a fourteen-day embryonic disc. )< 15. HF, head fold;
6 Ventral view of a fourteen-day embryonic disc. )< 15. HF, head fold; G, medullary groove; 8, mesoblastic somite; SP, segmental plate.
G, medullary groove; 8, mesoblastic somite; SP, segmental plate.


7 Dorsal view of a slightly curled fourteen-day embryo, showing further
7 Dorsal view of a slightly curled fourteen-day embryo, showing further
Line 1,278: Line 640:




===Plate 2===


PLATE 1
9 Embryo of fifteen days, showing the folding of the medullary plates. X 7. MF, medullary folds; 0, heart; 8, somite; LP, lateral plate; SP, segmental plate; L, allantois.
 
 
375
PLATE 2
EXPLANATION or fiGURES
 
9 Embryo of fifteen days, showing the folding of the medullary plates. X 7.
MF, medullary folds; 0, heart; 8, somite; LP, lateral plate; SP, segmental
plate; L, allantois.


10 Dorsal -View of late fifteen-day embryo. X 10. MC, mesencephalon;
10 Dorsal -View of late fifteen-day embryo. X 10. MC, mesencephalon; H0, rhombencephalon; V17, vitelline vein.
H0, rhombencephalon; V17, vitelline vein.


11 Lateral View of fifteen—day embryo. X 8. F0, prosencephalon; OP‘, optic
11 Lateral View of fifteen—day embryo. X 8. F0, prosencephalon; OP‘, optic vesicle; GA, branchial arch; LF, lateral fold.
vesicle; GA, branchial arch; LF, lateral fold.


12 The sixteen-day embryo. )( 10. 0, otocyst; I, lens vesicle; I V, optic cup;
12 The sixteen-day embryo. )( 10. 0, otocyst; I, lens vesicle; I V, optic cup; U, umbilical cord. '
U, umbilical cord. '


13 The seventeen-day embryo. X 6. F, oralafossa; GS, branchial clefts.
13 The seventeen-day embryo. X 6. F, oralafossa; GS, branchial clefts.


14 The eighteen-day embryo. X’ 4:. AL, fore limb bud; PL, hind limb bud;
14 The eighteen-day embryo. X’ 4:. AL, fore limb bud; PL, hind limb bud; MY, myelencephalon ; IS, isthmus; PF, pontine flexure; J, endolymphatic duct; OF, choroid fissure; DI, diencephalon; OP, olfactory pit; T0,; telencephalon; X, maxillary process.
MY, myelencephalon ; IS, isthmus; PF, pontine flexure; J, endolymphatic duct;
OF, choroid fissure; DI, diencephalon; OP, olfactory pit; T0,; telencephalon;
X, maxillary process.


15 The nineteen-day embryo. X 4. VE, ventricular portion of heart; AT,
15 The nineteen-day embryo. X 4. VE, ventricular portion of heart; AT, atrial portion of heart; K, mandibular process; CH, cerebral hemisphere.
atrial portion of heart; K, mandibular process; CH, cerebral hemisphere.


16 The twenty-day embryo. )( 4. II, myotome; Z, sclerotome; Q,infundibulum.
16 The twenty-day embryo. )( 4. II, myotome; Z, sclerotome; Q,infundibulum.




PLATE 2
===Plate 3===


PLATE 3
17 Longitudinal section through a fifteen-day embryo, showing the allantois. L, mesodermal portion of the allantois; LD, endodermal divertieulum of allantois; Y, yolk sac; HG, hindgut; A, amnion.


18 Transverse section through the elevervday blastocyst in the uterine lumen. UM, uterine mucosa; D, embryonic disc.


17 Longitudinal section through a fifteen-day embryo, showing the allantois.
19 Transverse section through a fourteen-day blastocyst. T, ectoplacental trophoblast; H, head process.
L, mesodermal portion of the allantois; LD, endodermal divertieulum of allantois;
Y, yolk sac; HG, hindgut; A, amnion.


18 Transverse section through the elevervday blastocyst in the uterine lumen.
20 Transverse section through the heart region of a fifteen-day embryo. M, medullary plate; 0, heart; PH, pharynx; N 0, notochord.
UM, uterine mucosa; D, embryonic disc.


19 Transverse section through a fourteen-day blastocyst. T, ectoplacental
21 Transverse section‘ through the somite region of a fifteen-day embryo. G, medullary groove; LP, lateral plate; MF, medullary fold; S, somite.
trophoblast; H, head process.


20 Transverse section through the heart region of a fifteen-day embryo.
M, medullary plate; 0, heart; PH, pharynx; N 0, notochord.


21 Transverse section‘ through the somite region of a fifteen-day embryo.
{{Footer}}
G, medullary groove; LP, lateral plate; MF, medullary fold; S, somite.
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Harman MT. and Dobrovolny P. The development of the external form of the guinea-pig (Cavia cobaya) between the ages of 11 days and 20 days of gestation. (1932) Amer. J Anat. 49(3) : 351-378.

Online Editor 
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This historic 1932 paper by Harman is the first in a series investigating the development of the guinea pig.



See also: Harman MT. and Dobrovolny P. The development of the external form of the guinea-pig (Cavia cobaya) between the ages of 11 days and 20 days of gestation. (1932) Amer. J Anat. 49(3) : 351-378.

Harman MT. and Dobrovolny P. The development of the external form of the guinea-pig (Cavia cobaya) between the ages of 21 days and 35 days of gestation. (1933) J. of Morphology, 54(3): 493-519.

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1914 Embedding Embryo | 1917 Oestrous cycle | 1921 Heart | 1932 Development Day 11-21 days | 1933 Development Day 21-35 | 1936 Genital and Endocrine | 1964 Oocyte EM



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The Development of the External Form of the Guinea—Pig (Cavia Cobaya) Between the Ages of Eleven Days and Twenty Days of Gestation

Mary T. Harman And Marjorie Prickett

Kansas State College Of Agriculture And Applied Science (1932)

Three Plates (Twenty-One Figures)


  • Contribution no. 136 from the Zoological Laboratory, Kansas State College of Agriculture and Applied Science.
  • The authors wish to express their indebtedness to Dr. H. L. Ibsen, of the Animal Husbandry Department, for his cooperation in furnishing the animals for the experiment and his assistance and advice regarding the feeding and care of them.


Introduction

Much research has been done on the embryology of the guinea-pig, but there remain many phases which have not been worked out.

Purpose

It has been our purpose to work out the development of those structures which determine the external form, as well as to supplement the works on the growth in weight and length of embryos of the guinea-pig between the ages of eleven days and twenty days of gestation. As our tables show, we have obtained many embryos of known copulation ages at intervals of every few hours throughout the day, thereby being able to study closely the slight but important changes occurring.

Review of Literature

Earlier investigators, Reichert (’61), Bischoff (352), Hensen (’76), Duval (’92), and Von Spee (’01), contributed chiefly to the knowledge of the development of the very young blastocyst and the formation of the fetal membranes. Recently, Maclaren (’26), Wilson ( ’28), and Hill and Sansom (’29) combined the contributions of these earlier workers with their own investigations and thus brought the development of the blastocyst up to about the tenth day. sDraper (’20) and Ibsen (’28) worked out the growth of the guinea-pig embryos as to weight and length. Ibsen used only those embryos twenty days and older at intervals of five days. Draper observed some few cases of the stages as early as ten days, but neither Draper nor Ibsen studied the structural development. Adloff (’04), Gruber (’06), Loeb (’O6), Weidekowich (’O7), Granzer (’08), Lohle (’13), Rabl (’13), Huber (’18), Dowd (’29), and others have contributed to the information concerning the development of tissues, organs, glands, etc.


Material and Methods

iAll of the animals used for this research were obtained through the cooperation and courtesy of the Animal Husbandry Department. The majority of the females, which were discardsfrom the genetics research, were secured when about a month old.

Breeding conditions

Conditions which might give rise to variations in the development of the embryos were eliminated as nearly as possible by keeping the breeding conditions uniform. The animals were kept in metal cages in the basement of a limestone building, so there was little range of temperature. A balanced ration of rolled—oats mixture, alfalfa hay, either sprouted cats or fresh green plants, and water was regularly provided the animals each day. Since the feeding was uniform and adequate, any differences in degree of development could hardly be interpreted on the basis of nutrition.


With a few exceptions, only young virgin females were used. These animals when secured were weighed, recorded, ear-tagged, and kept in cages separate from the males. At a regular time each day each female showing an open vagina was removed to a cage with a male and closely observed. If copulation occurred, the vagina was examined for the presence of a vaginal plug. In each case the record was made on the female’s card and in the general record book, as, for instance: (C-6 9 X A-1 8 ) 8.00 -A.M.,. December 2, 1929; vaginal plug. Thus a double check was kept which eliminated chances for error. This was desirable because the age of each litter used was in every case the exact copulation age calculated from the time of copulation to the time of killing the female. A


As a result of these careful observations only a few nonpregnant females were sacrificed. A total of ninety-four females was killed. Of the sixty-eight which showed a vaginal plug after copulation, sixty-one were found to be pregnant. Of the four showing no vaginal plug, two were pregnant and two were non-pregnant. Twenty-two cases were uncertain and of that number only nine Were pregnant.

Procedure

Before killing a female for dissection, all supplies were in readiness so that no delays need occur in removing the embryos. The female was again Weighed and then killed. The use of illuminating gas was found to be the most satisfactory Way to kill the animal.


The entire uterus with its contents was left in place in the animal until a drawing had been made showing the relative size and location of each embryo in the uterus. Embryos in the right uterine horn were indicated as R 1, R 2, R 3, etc., from the union of the horns distally to the fallopian tubes. In a similar manner in the left horn the embryos were accordingly designated as L 1, L 2, etc.


By cutting the uterine Wall carefully along the antimeso-e metrial side, it was possible to remove each embryonic blastocyst intact along with the deciduate maternal tissue.


Before further dissecting the blastocyst it was carefully weighed on an analytical balance, sensitive to 0.001 gram. Embryos of sixteen days and older were removed from the blastocyst, Weighed, measured by means of a vernier caliper, and then fixed in Bouin’s fixative solution. Lastly, the remaining fluid and the membranes Were Weighed separately. In the case of embryos younger than fifteen days the total Weight of the blastocyst with the deciduate tissue was so small that it was found to be impractical to Weigh the embryo, fluids, and membranes separately. Some fifteen-day embryos were Weighed apart from the rest of the blastocyst, but the chance for error on such small Weights is as great as the significance of the Weight.

Data and Observations

In tables 4 and 5 the majority of the litters obtained are recorded as to the age of the litter, the location of each blastocyst in the uterus, the Weight and length of the embryos, and the Weights of the membranes and fluids.


Location in the uterus

A total of 213 embryos was obtained from seventy-one litters. Reference to the table shows that there is a rather uniform distribution in the two horns of the uterus. The combinations most frequently found are ‘two right-one left’ and ‘one right-two left.’


TABLE 1

The distribution of the blastocysts within the uterus, showing the number of litters in which each combination occurred

NUMBER or ' INDIVIDUALS IN nmrvmusns IN LITTEB3 morn: HORN LEFT HORN

1 1 0

1 0 4

1 5 1

2 1 3

2 3 1

4 2 0

5 3 0

6 0 3

7 2 2

9 1 1

13 1 2

20 2 1 Total, 71 113 100

Size of the ova

There is a gradual increase in the weight of the total blastecyst and deciduate maternal tissue. The total weight of each was not taken in every case, for occasionally the membranes were broken, allowing the amnionic fluids to escape. Since the fluids have a high specific gravity, the loss of these would aifect greatly the weight of the total.


It will be noted from our data that in no instance does the weight of the heaviest ovum at any one age closely approach the weight of the heaviest ovum of the succeeding day. However, the weight of the lightest ovum at any one age may fall within the range of weights of the preceding day. The data in table 2 show that the average Weight of the thirteen-day ovum is less than that of the twelfth day. No conclusions may be drawn from this, for the number of cases recorded for those days was small. On the whole, though, there is an increase in Weight from day to day, so that in the ten-day period which We have studied the weight has increased itself almost ten times.


TABLE 2

The weight of the smallest ovum and the heaviest ovum for each age; also the average weight of all ova secured for each age. These figures include the weight of the deciduate maternal tissue

_ RANGE OF WEIGHTS AGE AVERAGE WEIGHT 'Lightest Heaviest Days Grams Grams Grams 20 1.685 1.460 1.935 19 1.325 1.015 1.625 18 ' 1.017 0.809 1.200 17 0.904 0.695 1.060 16 0.662 0.300 (resorbed?) 0.890 15 0.614 0.340 0.800 14 0.477 0.200 0.735 13 0.375 0.252 0.650 12 0.389 0.340 0.455 11 0.197 0.147 0.250 TABLE 3

The weight and length of the embryos for each given age, including the average, the smallest, and the largest of the group for each age

WEIGHT IN GRAMS LENGTH IN MILLIMETEES AGE, DAYS ‘ Average Minimum Maximum Average Minimum Maximum 20 0.1385 0.080 0.200 9.1 8.0 10.2 19 0.1183 0.060 I 0.245 8.25 6.9 9.7 18 0.0649 0.014 0.095 6.37 5.7 8.1 17 0.0541 0.025 0.107 5.38 4.2 6.7 16 0.0487 0.010 0.075 4.41 4.0 4.9 15 0.0116 0.005 I 0.015 3.73 1.9 _ 5.4

Accompanying this gradual increase in the total Weight of the blastocyst and deciduate membranes combined there is a corresponding increase in the Weight and length of the embryo alone. In table 3 it is to be noted that the average Weight of the embryo alone is 0.0116 gram at fifteen days as contrasted with the average Weight of 0.1385 gram at twenty days. There is a comparatively Wide range between the minimum and maximum weight and length of a given age, but this wide range does not alter the increase in the average.


Likewise there is the lengthening of the embryo from 3.73 mm. at fifteen days to 9.1 mm. at twenty days. This, as we shall see later, is not the true increase in length, for the shape of the embryo is undergoing such rapid changes that a true measurement of the length on the same basis can hardly be made.

Variations in development

It is evident that there is some ,variation in the weight of the total blastocyst or the embryo alone, for any given age. These variations may be due to one or more causes. An actual difference in development may occur for the same copulation age, for it is generally accepted that not always the same period of time elapses between copulation and fertilization- At this early stage an hour’s difference in the fertilization age may result in a great difference in the size as well as the appearance of the embryo. Also, some of the variation undoubtedly can be accounted for by mechanical dififerences, i.e., varying amounts of fluids adhering to the materials weighed or varying amounts of membranes remaining attached to the embryo. Embryos at this stage are not at all firm, so that when the length is taken a slightly different degree of curvature may present itself with each embryo. Thus, small diflerences in lengths may be obtained from embryos which are approximately the same actual length. But in spite of these variations, it is self-evident that there is a continual, more or less gradual, development of the ovum throughout the period.

The fetal membranes

In the youngest ova with the decidua which we have removed from the uterus, the placentaleside presents an appearance diifering little from the free end. In the very early eleven-day ovum there may be present still the blood clot marking the fusion of the decidua capsularis over the implanting ovum. In that case the placental pole may be distinguished as being at the end opposite the blood clot. Usually at the eleven-day stage the blastocyst with its membranes is a slightly oblong vesicle, with the long axis running through the placental pole. Examination of the blastocyst alone shows that it is an elongated closed cylinder (fig. 1) lying in the implantation cavity. At the one end, the placental pole, the thickened ectoplacental trophoblast has formed scattered villi which may be separated easily from the maternal tissue. The incomplete ‘inverted’ yolk sac is continuous with the ectoplacental trophoblast. Within the yolk sac at one end, the obplacental pole, is the ectodermal amnioembryonal mass, which by this time has arranged itself in the form of a hollow sphere of cells one layer thick. That portion of the sphere which is toward the placental pole will give rise to the ectoderm of the amnion; the remainder is destined to give rise to the ectodermal parts of the embryo.


By the thirteenth day (fig. 2) the blastocyst can be separated from the decidua only with difiiculty, for the villi have continued to form at the placental pole until the ovum is becoming rather well implanted. The blastocyst is tending to widen and flatten (fig. 3). The amnio-embryonal area at thirteen days is well organized into two portions, the amnion and the ectoderm of the embryonic disc. Toward the end of the thirteenth day the anlage of the allantois appears as a spongy mesodermal mass at the posterior end of the embryonic disc.


The allantois enlarges and becomes more spongy, so that by the fifteenth and sixteenth days it is almost as large as the embryo itself. By the seventeenth or eighteenth day it has assumed a more stalked appearance and later it fuses with the placental area to form the allantoic placenta. Lying close beside the allantoic stalk are the umbilical vein and umbilical arteries carrying blood from and to the placenta. Although the allantois attains such an extended degree of development, it is practically devoid of endoderm. The endo dermal diverticulum is so short that there is little or no endoderm out of the body (fig. 17).


The yolk sac of the guinea-pig is peculiar in that it forms not a closed vesicle as in most other mammals, but an ‘inverted’ vesicle, presenting the endodermal surface to the cavity of the uterus, and at the edge distally from the embryonic disc it is fused with the ectoplacental trophoblast (fig. 18). However, it nowhere forms a villous attachment. The circulation of the yolk sac is carried on through the vitelline veins and arteries. Thereis no crossing over of blood vessels from the yolk sac to the placenta and vice versa, for in the full-term fetal membranes the yolk sac may be stripped from the placenta with little or no resulting hemorrhage.


A thinning of the decidua capsularis and a thickening of the decidua basalis enable one to distinguish quickly and easily the placental area. By the nineteenth or twentieth day the embryonic placenta can be separated from the maternal placenta.


Thus, in the ten-day period which we have studied all the fetal membranes have become well established and have become functional.

Early stages of differentiation

As before stated, in the blastocyst of the eleven-day stage not only the primary germ layers are established, but the yolk sac and ectoplacental trophoblast are rather well defined and the ectodermal amnio-embryonal mass has split into a hollow sphere of cells (fig. 1). A section through the elevenday blastocyst (fig. 18) shows the mesodermal cells pushing outward along the endoderm of the yolk sac; the mesoderm already lies in a thin layer between the ectoderm and endoderm in the region of the embryonic disc.


Externally at thirteen days, the amnio-embryonal area is seen to consist of the embryonic disc and the amnionic portion. Early in the thirteenth day (fig. 2) the embryonic disc appears as a dome-shaped plate of cells consisting of all three germ layers, of which the endoderm, a part of the ‘inverted’ yolk sac, lies externally. Within the cavity between the embryonic disc and the placental portion is the amnion lying rather close to the dorsal surface of the embryonic disc.

During the thirteenth day (fig. 3) a rapid proliferation and growth dorsalward of mesodermal cells occur at the posterior end of the embryonic disc marking the anlage of the allantois.

The fourteen-day embryo

The study of the embryo itself really begins with the embryo of the fourteenth day (fig. 4), for at the fourteenth day the embryonic disc has thickened and formed definite limits. Along the median line, in the posterior half of the embryonic disc, is a narrow strip, the primitive streak. This appears lighter and thinner than the surrounding area, due to the presence of the primitive groove in the primitive streak. Marking the anterior end of the primitive streak is a somewhat thickened area, Hensen’s node. Laterally the germ layers are thickened, especially the ectoderm. However, this thickening does not extend quite to the edge of the embryonic disc; thus the area has a wing-like appearance. Anterior to Hensen’s node and lying between and ventral to the medullary plates is the prominent club—shaped head process (fig. 19).


Further development during the fourteenth day (fig. 5) gives the embryo a difi"erent appearance. The head process no longer occupies such a prominent position; in fact, it is not noticeable at all in the whole mount. The primitive streak has shortened and the primitive groove has almost disappeared. Hensen’s node appears as scarcely more than merely the end of theprimitive streak. In the anterior and lateral portions of the disc the ectoderm. is thickening decidedly to form the broad flat medullary plates.


These medullary plates at a slightly further development (fig. 6) increase in length much more than in width, and as they do so the medullary groove is formed between them. The head fold, as a very shallow crescentic groove, is beginning to raise the head from the embryonic disc. Ventrally 361

TABLE 4

A tabulation of the length and weight of individual embryos within litters of succeeding ages between fifteen and twenty days, inclusive, and the location of each embryo within the uterus

TIME ELAPSING : INDIVIDUALS IN LITTER

BETWEEN _ Right horn ! Left horn

COPULATION AND REMOVAL LITTER I R 3 R 2 R 1 ' L 1 L 2 L 3

N0.

Weight I Length Weight Length Weight Length Weight Length Weight Length Weight Length 3 of ' of of of of of of of 1 of ' of of of embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo embryo

.200 9.9 .150 9.3 ' .185 8.6 .135 9.4 .160 10.2 .165 9 .080 8 .100 8.8 .110 9.7 .100 8.3.

.100 7.3 ' .060 7.6 .110 .110 9.1 .245 9.7 .155 9 .115 9.1 .110 8.6 .130 .115

= C\‘Io3 °°“."'7"':'

.090 7.3

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.150 8 9 .140 .105 7.6 .070 .080 7 8 .100

.120 8.2

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.080 .080 6.9 .110 .145 8.2 .110 .130 .055 .070 .060

ymmmommnmmmn

.090 .160

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.140 8.7" .125 8.8 .075 6.5 ‘ .060 6.1

‘*3

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.075 .0 75 .014 .095

‘W .060

.065 .065 .045 .090

.040 .107 .025 .0 70 .035 .045 .045 .064 .055

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A h: <$‘—‘u'.5Lr'.>b- to

.040 .059 .025 .089 .045 .045 .065

C I

(Resorbed) (Resorbed)

.090 .050

|.f'.‘IlQ[_~... heme: Q<Hm©<<O

.035 4.3

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.055 .060

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.060 .070

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.035 .010

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.005 362

TABLE 5

A tabulation of the weight of the blastoeyst with the adhering maternal tissue and the weight of the same after the embryo and the amnionie fluid have been removed. These are the fetal membranes and the adhering maternal tissue of the embryos listed in table 4 and additional blastoeysts of eleven to fourteen days, inclusive

TIME ELAPSING INDIVIDUALS IN LITTER

BETWEEN . COPULATION AND Rlght horn . Left horn

REMOVAL R3 R2 R1 L1 L2 L3

LITTER N0.

Days Hours

'.1s.£oo1s's1q J0 attfizm. Eeuuxqmam

0 '-11{3!9M qsfiooqselq J0 ’-11I3!9A\ B8II‘B.IqIII9III 50 1113181321 qsuiaoasnlq

0 11I3E9AA S9(IB.Iq‘[1I3II.'I

0 mfizam qs.&oo1ss[q

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this groove produces an internal fold in the endoderm as well as the other germ layers; this internal fold of the endoderm is the first appearance of the foregut. Medially throughout most of the length of the embryonic disc a longitudinal split occurs in the heretofore continuous plate of mesoderm. The segmental plates have been formed by a proliferation of the mesodermal cells at either side of this median slit. From these plates then in turn the first pair of mesodermic somites arises a little posterior to midway in the embryonic disc. The number of somites increases to as high as six pairs by the end of the fourteenth day (fig. 7). Lateral to the meso—~ dermic somites are the lateral plates. At the edge of the head fold the vitelline veins are forming in the splanchnic mesoderm. The head fold is deepening and extending farther posteriorly and in so doing it allows the head, consisting chiefly of the medullary plates and foregut, to curl up dorsally into the amnionic cavity. The medullary plates have continued to thicken, especially anteriorly, and are beginning to be elevated as a pair of folds in the region just anterior to the first mesodermic somite.

The fifteen-day embryo

Early in the fifteenth day (fig. 8) probably the most striking thing about the embryo is the broad flat medullary plates, which are so greatly thickened. Anterior to the region of the vitelline veins (fig. 20) the plates are so fiat that there is hardly an indication of the folding except at the most anterior ‘ear~like’ parts, which are really the extreme edges of the plates beginning to fold. From the region of the vitelline veins and proceeding posteriorly (fig. 21) there is a deep groove between the medullary folds, especially in the anterior somite region. However, the plates have not yet approached each other closely enough to fuse.


The folding and differentiation of the splanchnic mesoderm, which had resulted in the vitelline veins at the fourteenth day, have continued and by the fifteenth day have given rise to the heart. At the beginning of the fifteenth day the two bulb-like myocardial foldsxare approaching each other preparatory to fusing into a single heart tube.


These folds fuse a little later (fig. 20) to form the single heart tube, which grows so rapidly that it begins to twist almost immediately. Simultaneously the folding of the medullary plates has been extended forward. As the folds have become farther extended theyphave also begun to fuse together, forming the medullary or neural tube in the region of the most anterior somites. The medullary folds terminate very abruptly at the anterior end, indicating a late closure of the anterior neuropore. The lateral plates have become much more distinct and show the location of the lateral folds. The number of somites continues to increase, but a difference of two or three somites does not in itself effect much change in appearance now.


The fusing together of the medullary plates proceeds anteriorly and posteriorly, converting the folds and groove into the medullary tube except for the extreme anterior end and the portion in the region of, and posterior to, the last two or three somites (fig. 9). Meanwhile, the walls of the neural tube have been growing more rapidly in some areas than in others, giving the neural tube an irregular bulging form. These bulges may be identified as the primary vesicles of the brain. The optic vesicles are formed as lateral evaginations of the walls of the anterior vesicle. This occurs some time before the medullary plates have fused dorsally.


The heart tube. in the late fifteen-day development has twisted to the right about as much as in the thirty-six-hour chick embryo. ‘When the heart first appeared, it was in the midbrain region, but now it has descended to a position ventral to the hindbrain. The first pair of branchial clefts, although not prominent, is distinguishable. By a continued folding together of the ventral body wall, the anterior part of the embryo is raised from the yolk. sac.


The sixteen-day embryo

The sixteenth day brings about the complete closure of the body Wall ventrally, except for the region of the umbilicus (fig. 12). This separation of the embryo from the extraembryonic membranes has been accomplished by the pinching together of the head fold, the lateral limiting sulci, and the tail fold. In the splanchnopleure the folds unite to form the alimentary canal, which is now in contact With the yolk sac only through the yolk stalk.


The ventralward bending of the head commencing late in the fifteenth day has increased to form the cephalic flexure. Due to the closing of the body wall, the embryo tends to lie on one side and assumes a curvature throughout its length.


The flexure and curvature of the neural tube correspond to those of the body already described. Continued difier— entiation of the Walls of the neural tube leads to the more definite establishment of the primary vesicles of the brain. The outer Wall of the optic vesicle which had begun as a lateral evagination from the prosencephalon late in the fifteenth day invaginates to form the double-Walled secondary vesicle, the optic cup. Ventrally the Wall of the cup is incomplete, leaving the opening known as the choroid fissure. An invagination of the thickened ectoderm external to the optic cup forms a thick-Walled pit, the lens vesicle. The auditory vesicle, the otocyst, appears as a pair of thickenings and invaginations of the ectoderm on the dorsolateral sides of the head a little anterior to the first somite. These pits deepen and the lips of the pits begin to fold together at the twenty-three—somite stage. At about this time the ectoderm in front of the optic vesicles begins to invaginate to form a pair of pits, the olfactory pits. Three or four branchial clefts may be present, but there is little indication of the maxillary and mandibular processes, hence the oral fossa is large and open.


Further increase in the number of somites, now ranging from sixteen to twenty-three pairs, decreases the length of the segmental plates.

The seventeen-day embryo

The somites now range in number from twenty-three pairs to twenty-nine pairs, and the segmental plates have shortened more (fig. 13). Four branchial clefts may still be present, but the third and fourth are no longer outstanding. In many seventeen-day embryos the fourth pair of clefts has been closed by the fusing together of the fourth and fifth pairs of arches. The first pair of arches begins to thicken, especially along the anterior border where the maxillary and mandibular processes are appearing. These processes are the primordia of the upper and lower jaws.


The cephalic flexure differs in appearance from the flexure at sixteen days only in that the angle is a little more acute. The general curvature of the body has increased slightly. The only change in the neural tube is the further thinning of the roof plate of the rhombencephalon. The complete separation of the otocyst from the outside ectoderm is effected during the seventeenth day.

The eighteen-day embryo

The curvature of the embryo is becoming more marked in the region of the first somites and in the region immediately anterior to the tail (fig. 14). These regions of increased curvature are the locations of the cervical or nape flexure and the sacral fiexure. It is not until well toward the end of the eighteenth day that the sacral and nape flexures become well established.


In addition to the flexures of the neural tube which are induced by the body flexures, there is the pontine flexure, which occurs in the neural tube alone. This third brain flexure makes its appearance as a ventral bending of the floor of the anterior portion of the hindbrain or rhombencephalon. Continued difierentiation of the neural tube initiates the development of the secondary vesicles. A forward and dorsalward expansion of the anterior end of the forebrain gives rise to the telencephalon, of which the most prominent parts are the primordia of the cerebral hemispheres. The remainder of the prosencephalon is the diencephalon. Very little change has occurred in the mesencephalon. Hardly any expansion occurs in the region between the hindbrain and the midbrain; this region is now known as the isthmus. Two divisions from the rhombencephalon arise, the metencephalon and the myelencephalon. The latter is characterized by the extremely thin roof.


The eye is assuming a more rounded appearance; the lens vesicle is separating from the outside ectoderm and is becoming a spherical body; the choroid fissure, although still present at the beginning of the eighteenth day, has almost disappeared by the end of that day.


Complete closure of the mouth of the otocyst converts the vesicle into a closed sac. A conical elevation of the external side of the most dorsal portion of the otocyst gives rise to the endolymphatic duct.


The anterior limb buds arise: as a result of the rapid proliferation of the mesenchymal cells in the region of the sixth to eleventh somites. Beginning proliferations of the mesenchyme and a slight swelling posteriorly indicate the hind limb buds, but as yet they are no more then slight ridges.

The nineteen-day embryo

The sacral and nape flexures which had become well defined by the end of the eighteenth day have reached their greatest development at nineteen days (fig. 15). No new structures in direct association with the neural tube originate during the nineteenth day. The optic vesicle is rapidly becoming much smaller in comparison to the size of the forebrain. The choroid fissure is no longer visible. Pigment is appearing in the iris and retinal layer of the vesicle.


The heart, last referred to in the fifteen-day embryo, has migrated posteriorly to lie in the body cavity in the region of the fore limb buds. Rapid elongation of the heart tube between the anterior and posterior fixed ends has produced a folding. During the nineteenth and twentieth days some portions of the tube expand more than others and thus there are established the divisions of the heart, of which only the atrial and ventricular portions are discernibleexternally. The ventricular portion lies somewhat ventral and posterior to the atrial portion.


The anterior limb buds, which were mere swellings in the eighteen-day embryo, have become well-rounded outgrowths. The hind limb buds are located in the region of the twentyeighth to thirty-third somites.


At the nineteenth day the last somites have been formed; the final number is forty—one pairs. From each anterior somite there have arisen the sclerotome and myotome, even before the last few somites have been formed.

The twenty-day embryo

Except for the few most posterior ones, all the somites have differentiated into the myotome, or a muscle plate, and the sclerotome, which will give rise to the vertebrae (fig. 16). Development of the neural tube during the twentieth day produces no new structures, but rather there is a further definition of the secondary vesicles already established. The eye, which is becoming relatively smaller in comparison to the size of the forebrain, shows much pigmentation. The limb buds are beginning to elongate to the extent that the distal end is free from the surrounding tissue. The heart presents an appearance similar to that at nineteen days.


Two pairs of branchial clefts remain unclosed. The maxillary and mandibular processes from the first pair of arches are enlarging and are growing ventralward, but as yet they have not united so as to form the face. Hence, the oral fossa is still wide and open. The olfactory pits are still rather widely separated.


During the period from the sixteenth day to the twentieth day the umbilical cord has been constricting until by the twentieth day its connection with the body covers only a small area.

Discussion

The foregoing data as to weight and length parallel those by Draper (’20). However, Draper took his data from a fewer number of cases and he recorded embryo blastocysts of only every second or third day. He gives the weight of three ova at eleven days as 0.425 gram (average, 0.142 gram each) as contrasted with our figure of 0.197 gram. Likewise, the weight he records at twenty days is not as heavy as we have found. On the other hand, his weights given for fourteen, fifteen, and seventeen days are heavier than we have secured. But since the weights dealt with were so small and since the cases recorded were so few, the difference between our data and those of Draper is of no significance. Moreover, Draper records only a very few cases falling within the range of ages of the embryos we have observed as to weight and length, so that a comparison with his results can hardly be made. Ibsen (’28) records data from embryos only twenty days of age and older, so no comparison can be made between these data and his.


The establishing of the germ layers and the development of the blastocyst up to about the tenth day have been worked out by earlier investigators. Recent investigations and interpretations by Maclaren (’26), Wilson (’28), and Hill and Sansom (’29) on the earlier phases of development, in addition to the contribution by Duval (’92) covering the fetal membranes, correspond to and substantiate our investigations and conclusions.


Reference to plates 1, 2, and 3 shows that in the ten-day period studied the embryonic tissue has developed from the blastocyst showing only the germ layers to the embryo closely approaching the degree of development of a fetus. Development of the structures such as the eye, ear, limb buds, and heart is similar to the development of those same structures in other mammals. On the other hand, the presence of the ‘inverted’ yolk sac, the extensive development of the allantois almost devoid of endoderm, and the extremely flat medullary plates anteriorly up to the fifteenth day are quite characteristic of the guinea-pig. The development of these structures in the other rodents is similar, but does not parallel exactly the development in the guinea-pig.

Summary

  1. The guinea-pig blastocyst presents the ‘inversion’ of the germ layers.
  2. The allantois arises on the thirteenth day as a mesodermal outgrowth, and by the eighteenth or nineteenth day it fuses with the ectoplacental trophoblast to form the allantoic placenta.
  3. The primitive streak appears late in the twelfth or early in the thirteenth day and has completely differentiated by the end of the fourteenth day.
  4. The medullary plates arise on the fourteenth day as broad flat plates; the plates fuse together first in the hindbrain region on the fifteenth day; the neural tube differentiates into the primary vesicles late on the fifteenth day, and difierentiates into the secondary vesicles on the eighteenth day.
  5. The first branchial arch is formed at fifteen days, at sixteen days the maximum number is present, and at twenty days three arches are still present.
  6. The otocyst appears at sixteen days, closes off at seventeen days, and shows the endolymphatic duct at eighteen days.
  7. The optic vesicle evaginates at fifteen days; the choroid fissure disappears at eighteen days; pigmentation begins at nineteen days.
  8. The first pair of somites is formed at fourteen days and the final number of forty-one pairs is formed at nineteen days.
  9. The body folds off from the yolk sac, except for the umbilical cord, at sixteen days.
  10. The limb buds arise at eighteen days.
  11. The cephalic flexure is found at fifteen days and the sacral and nape flexures at eighteen days.
  12. The weight increases from 0.0116 gram at fifteen days to 0.1385 gram at twenty days; the length increases from 3.73 mm. at fifteen days to 9.1 mm. at twenty days.


Bibliography

ADLOFF, P. 1904 "Ueber den Zahnvvechsel von Cavia cobaya. Anat. Anz.-, Bd. 25, S. 141-147.

AS-SHETON, R. 1895 A re-investigation in the early stages of thedevelopment of the rabbit. Quart. Jour. Mic. Sci., vol. 37, pp. 113--164.

1895 On the causes which lead to the attachment of the mammalian embryo to the Walls of the uterus. Quart. J our. Mic. Sci., vol. 37, pp. 173-190.

BISCHOIPF, T. L. W. 1852 Entvvicklungsgeschichte des Meerschweinchens. Giessen.

Down, D. R. 1928 The development of the ovary of the guinea-pig, Cavia cobaya, in embryos of eighteen to thirty days of age inclusive; with some observations concerning its subsequent development. Thesis, K. S. C.

DRAPER, R. L. 1920 The prenatal growth of the guinea-pig. Anat. Rec., vol. 18, pp. 369-392.

DUVAL, M. 1892 Le placenta des rongeurs. Jour. de l’Anat. et de la Phys., T. 25, pp. 309-342, 573-627; T. 27, pp. 24-73, 344-395; T. 28, pp. 58-98, 333-453.

GANZER, H. 1908 Anatomic und Entwicklung des Gebisses vom Meerschweim chen (Cavia cobaya). Berlin.

GRUBER, C. 1906 Bau und Entwicklung die ausseren Genitalien bei Cavia cobaya. Morph. Jahrb., Bd. 36, S. 3-26.

HEAPE, W. 1883 Development of the mole. Quart. Jour. Mic. Sci., vol. 23, pp. 157-174.

HENSEN, V. 1876 Beobachtungen iiber die Befruchtung und Entwicklung des Kaninchens und Meerschweinchens. Zeit. fiir Anat. und Entwick., Bd. 1, S. 213-273, 353-423.

HILL, J . P., AND G. S. SANSOM 1929 Observations on the structure and mode of implantation of the blastocyst of Cavia. Jour. of Anat., vol. 64, PP.~ 113-115.

HUBER, G. C. 1918 On the anlage and morphogenesis of the chorda dorsalis in Mammalia, in particular the guinea—pig (Cavia cobaya). Anat. Rec., vol. 14, pp. 217-264.

IBSEN, H. L. 1928 Prenatal growth in guinea.-pigs, with special reference to environmental factors affecting Weight at birth. J our. Exp. Zo5l., vol. 51, pp. 51-91.

JENKINSON, J . W. 1900 A reinvestigation of the early stages. of the development of the mouse. Quart. Jour. Mic. Sci., vol. 43, pp. 61-82.

LEE, T. G. 1903 Notes on the early development of rodents. Am. J our. Anat., vol. 2, pp. x-xi (abstract).

LOEB, LEO 1906 Ueber die Entwicklung des Corpus luteum beim MeerschWeinchen. Anat. Anz., Bd. 28, S. 102-106.

LOHLE, B. 1913 Die bildung Gaumens bei Cavia cobaya. Morph. Jahrb., Bd. 46, S. 595-654.

MACLAREN, N. 1926 Development of Cavia; implantation. Trans. Roy. Soc. Edin., vol. 55, p. 115.

RABL, H. 1913 Die Entwicklung der Derivate des Kiemendarmes beim Meerschvveinchens. Arch. mikr. Anat., Bd. 82, S. 74-147.


REICHERT, C. B. 1861 Beitriige zur Entwicklungsgeschichte des Meersehweinchens. Phys. Abh. Akad. Wiss., Berlin, S. 97-216.

ROBINSON, A. 1904 Lectures on the early stages in the development of mammalian ova and on the formation of the placenta "in the difierent groups of mammals. J our. Anat. and Physiol., Vol. 38, pp. 186-204, 325-340, 485-502.

SPEE, GRAF V. 1901 Vergange bei der Implantation des Meerschweinehen Eies in der Uteruswand. Anat. Anz., Bd. 12, S. 131-136. 1883 Beitrage zur Entwicklungsgeschichte der friiheren Stadien des Meerschweinchens bis zur Vollendung der Keimblase. Arch. fur Anat. und Physiol.

I Srocsann, O. R., AND G. PAPANICOLAOU 1919 The vaginal closure membrane, copulation, and the vaginal plug in the guinea-pig, with further considerations of the oestrous rhythm. Biol. Bull., vol. 37, pp. 222-245.

WIDAKOWICH, V. 1907 Ueber Entwicklungsdifierenzen des Zentralnervensysterns drier gleichaltiger Embryonen V011 Cavia eobaya. Arb. Ne-urol. Inst. Wien, Bd. 16, S. 452-468.

WILSON, J. T. 1928 On the question of the interpretation of the structural features of the early blastocyst of the guinea—pig. J our. Anat., vol. 62, pp. 346—358.


Explanation of Plates

The outlines for the drawings were made with the aid of a microprojector apparatus, so the proportions of the drawings are an exact reproduction of the proportions of the embryos drawn. The figures of plate 3 are photomicrographs.

Plate 1

EXPLANATION or fiGURES

1 The eleven-day blastocyst. X 10. E, ectodermal amnio—embryona1 mass; Y, yolk sac; V, villi; T, ectoplacental trophoblast.

2 Early thirteen-day blastocyst. )( 10. D, embryonic disc; A, amnion.

3 Late thirteen—day blastocyst. )( 10. L, allantois; B, extra-embryonic blood

vessels. 4 Early fourteen-day embryonic disc with the adjacent yolk sac. X 10.

H, head process; N, Hensen’s node; P, primitive streak.

5 The fourteen-day embryonic disc, showing a little further development.

)< 15. M, medullary plate.

6 Ventral view of a fourteen-day embryonic disc. )< 15. HF, head fold; G, medullary groove; 8, mesoblastic somite; SP, segmental plate.

7 Dorsal view of a slightly curled fourteen-day embryo, showing further

development. )< 15. V17, vitelline vein; MF, medullary fold; LP, lateral plate.

8 Early fifteen-day embryo. )( 15.


Plate 2

9 Embryo of fifteen days, showing the folding of the medullary plates. X 7. MF, medullary folds; 0, heart; 8, somite; LP, lateral plate; SP, segmental plate; L, allantois.

10 Dorsal -View of late fifteen-day embryo. X 10. MC, mesencephalon; H0, rhombencephalon; V17, vitelline vein.

11 Lateral View of fifteen—day embryo. X 8. F0, prosencephalon; OP‘, optic vesicle; GA, branchial arch; LF, lateral fold.

12 The sixteen-day embryo. )( 10. 0, otocyst; I, lens vesicle; I V, optic cup; U, umbilical cord. '

13 The seventeen-day embryo. X 6. F, oralafossa; GS, branchial clefts.

14 The eighteen-day embryo. X’ 4:. AL, fore limb bud; PL, hind limb bud; MY, myelencephalon ; IS, isthmus; PF, pontine flexure; J, endolymphatic duct; OF, choroid fissure; DI, diencephalon; OP, olfactory pit; T0,; telencephalon; X, maxillary process.

15 The nineteen-day embryo. X 4. VE, ventricular portion of heart; AT, atrial portion of heart; K, mandibular process; CH, cerebral hemisphere.

16 The twenty-day embryo. )( 4. II, myotome; Z, sclerotome; Q,infundibulum.


Plate 3

17 Longitudinal section through a fifteen-day embryo, showing the allantois. L, mesodermal portion of the allantois; LD, endodermal divertieulum of allantois; Y, yolk sac; HG, hindgut; A, amnion.

18 Transverse section through the elevervday blastocyst in the uterine lumen. UM, uterine mucosa; D, embryonic disc.

19 Transverse section through a fourteen-day blastocyst. T, ectoplacental trophoblast; H, head process.

20 Transverse section through the heart region of a fifteen-day embryo. M, medullary plate; 0, heart; PH, pharynx; N 0, notochord.

21 Transverse section‘ through the somite region of a fifteen-day embryo. G, medullary groove; LP, lateral plate; MF, medullary fold; S, somite.


Cite this page: Hill, M.A. (2024, May 20) Embryology Paper - Guinea pig development 11 to 21 days. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Guinea_pig_development_11_to_21_days

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