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The placenta of the four-eyed opossum (philander opossum)Philander opossum).

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The Placenta of the Four-eyed Opossum
(Philander opossum) '
Department of Anatomy, Washington University School of Medicine,
St. Louis, Missouri and Department of Biology, Swarthmore College,
Swarthmore, Pennsylvania
The placental membranes of the four-eyed opossum were studied by
light and electron microscopy. The individual fetuses in each uterus were surrounded
by amnion, had allantoic sacs of approximately the same size as each fetus, and
were situated in a common yolk sac cavity. The extent of the choriovitelline placenta
was marked by a prominent sinus terminalis, and at this margin there was a region
where the trophoblast cells penetrated folds of the endometrium. Elsewhere the
choriovitelline placenta was closely applied to the uterine epithelium along most of
its surface, but the microvilli of the two epithelia did not interdigitate. Numerous
inclusion bodies were seen in the trophoblast of both the choriovitelline and bilaminar
omphalopleure portions of the placenta, but the aggregates were larger in the latter.
The endoderm celIs of the choriovitelline placenta had extensive endoplasmic reticulum and numerous mitochondria, but did not have conspicuous absorption canaliculi.
Placentation in the four-eyed opossum appears to represent a progressive advance
over that of the Virginia opossum both in confluence of the yolk sacs of the fetuses
and in having a region of penetration of the maternal endometrium by trophoblast.
Only two families from two superfamilies (Simpson, '45) or suborders (Romer,
'66) of marsupials are currently found in
the Western Hemisphere. Although information is slowly accumulating concerning reproduction in many of the species of
New World marsupials (R. Enders, '35,
'66; Biggers, '66, '67: Phillips and Knox
Jones, '68), the only comprehensive histological study of placental structure of a
New World marsupial is that of the North
American opossum, Didelphis marsupialis
uirginiana (McCrady, '38). Placentation
in the Australian marsupials has been
more thoroughly studied and has recently
been reviewed by Sharman ('61). Nevertheless, no studies of the fine structure of
the choriovitelline placenta of marsupials
have yet appeared.
We recently had an opportunity to obtain placentas from E. pouched four-eyed
opossum (Philander qpossum) under unusually good field conditions, so that material could be obtained for both light and
electron microscopy. In addition, reproductive tracts from a number of female
four-eyed opossums collected in Nicaragua
were made available to US? Several differences in placentation between Philander
and Didelphis and some of the cytological
ANAT. REC., 165: 431-450.
features of the placenta of Philander are
reported in this study.
An adult female four-eyed opossum
caught in Panama was taken to the laboratory on Barro Colorado Island. When
the animal was killed by cervical dislocation, it was found to have a well-developed
pouch and prominent mammae. Examination of the uteri showed that both contained several embryos. The right uterus
was opened, and parts of the placentas
from the three fetuses (average 7 m m
CRL) were fixed in 0.1 M phosphate-buffered glutaraldehyde at pH 7.3. The other
uterus was placed whole in Bouin's fluid;
then a small slit was made at one end to
allow entry of the fixing fluid. Some of the
pieces of the placenta fixed in glutaraldehyde were rinsed in buffer and post-fixed
in buffered 2% osmium tetroxide. These
pieces of tissue were then dehydrated in
alcohol, passed through toluene, and
Received April 2, '69. Accepted May 19, '69.
ISupported by grant 5 R01 HD02613 from the
National Institute of Child Health and Human Development.
2 This genus is sometimes listed as Metachirops.
See Hershkovitz ('49) as referred to by Hall and
Kelson ('59), for a dis;ussion of the generic names.
3 We are grateful to Dr. John Biggers, who lundly
put this material at our disposal.
43 1
placed in Araldite epoxy resin (Durcupan) .
The blocks were polymerized over a drying
light for two days. The rest of the glutaraldehyde-fixed material was transported
to St. Louis, where it was similarly postfixed, dehydrated and embedded after five
days in glutaraldehyde. Sections 2 v thick
were made from the plastic-embedded material and stained with azure B for examination by light microscopy. Thin sections
were stained with lead citrate before examination in the electron microscope.
The Bouin-fixed uterus was opened and
the cranial half carefully dissected. The
other half was serially sectioned for light
microscope study. This uterus contained
five embryos and their placentas. Three of
the reproductive tracts from the material
collected in Nicaragua contained embryos.
One of these was judged to be an advanced
stage of resorption and was not used.
The second was an early stage, prior to
penetration of the egg shell membrane. The
third was at the stage of parturition with
only one of the fetuses still in the uterus.
Sections of the placenta of this uterus
were used to confirm the general histological relationships seen in the more complete conceptuses from Panama, but, except as noted, the morphological description is based on the latter material.
The periodic acid-Schiff method, using
diastase extraction as a control, was used
to demonstrate glycogen on some of the
sections of the Bouin-fixed material, Perl's
method was used to demonstrate iron, using small pieces of glutaraldehyde-fixed
material which had been embedded in
paraffin without post-fixation.
Arrangement of fetal membranes
The foureyed opossum fetuses were surrounded by their individual amniotic sacs
within a common yolk sac cavity in each
uterus (figs. 1,2,3). The fetuses had welldeveloped digits on their forelimbs, but
incomplete facial development. The choriovitelline placenta (vascular yolk sac and
trophoblast) of each fetus occupied a distinct discoidal area of the surface of the
uterus and was delimited at its margins
by the sinus terminalis (figs. 1, 4). Lateral to the choriovitelline placenta was the
bilaminar amphalopleure (avascular yolk
sac and lrophoblast). The bilaminar
omphalopleiure was rather loosely associated with epithelium and, in two places,
was separated by residual shell membranes
from the uterine epithelium (fig. 5). The
choriovitelline placenta was more closely
associated vvith the uterine epithelium and
resisted displacement from it, especially at
the margins of the disc.
The allantois was large (approximately
the size of the fetuses) and vascular, but
was invested by an avascular portion of
the yolk sac so that it did not contact the
choriovitelline placenta (fig. 4). As in
the Virginia opossum, the mesoderm did
not completely surround the amnion, and
the extraembryonic celom was small.
The bilanninar omphalopleure and choriovitelline placentas covered almost the
entire luminal surface of each uterus.
These bilaniinar and trilaminar structures
are relatively uniform, but the margins of
the choriovitelline placenta show a number of modifications from either of the two
major regions and should be considered a
separate zone.
Bilaminar omphalopleure
The trophoblast in the bilaminar
omphalopleure consists of large, roughly
cuboidal irregular cells containing areas
of included material (fig. 9). They have
numerous long microvilli in clumps irregularly over their apical surface. The basal
surface also has a number of processes.
The mitochondria are rod-shaped but are
not abundant. Short stretches of granular
endoplasmic reticulum are present but occupy only a small portion of the cytoplasm.
The apical junctional complexes between
trophoblast cells are extensive, and desmosomes are abundant along the lateral
borders. Frequently there is an area of
close apposition at the basal as well as the
apical end of the cells. The inclusions of
numerous large vesicles containing debris
of different densities, including crystalline
material, ace the most striking cytological
feature of these cells.
The endodermal cells which underlie
the trophoblast in this area tend to be
Fig. 1 Diagrammatic representation of a cross-section of a pregnant uterus of the four-eyed opossum. The trophoblast is heavily stippled; endoderm is a solid black line; the vessels of choriovitelline placenta are solid black ovals; the vessels of the allantois are small circles. Note that a large
vessel, the sinus terminalis, marks the edge of the two choriovitelline placentas shown, and that the
trophoblast penetrates the uterine epithelium a t the tips of endometrial folds near the margin. In the
lower left, a portion of the residual shell membrane is folded between the trophoblast and the endometrium. On the right side is a small residual portion of the fused bilaminar omphalopleures which
once separated the embryos.
squamous to low cuboidal with numerous
irregular projections. They are relatively
devoid of inclusions and are cytologically
The structure of the bilaminar omphalopleure is similar both where it is loosely
applied to the epithelium and where it is
folded back upon itself in small pockets.
However, at one place in the serially
sectioned portion of the uterus, there
was a highly folded double section of
bilaminar omphalopleure recoiling into
the yolk sac cavity. The degenerate nature
of this region and the double folding led
to the conclusion that it was residual ruptured fused bilaminar omphalopleure from
adjacent fetuses.
cupy scattered areas close to the center
of the trophoblast. Small individual strands
of granular endoplasmic reticulum are
abundant, as are polyribosomes not associated with membranes. The basal regions
of the trophoblast cells show numerous
indentations into which microvilli project.
Thus both surfaces of the trophoblast cell
are extensive in area. Regions of fine filaments are often associated with the basal
portions of the cell.
Interposed between the trophoblast and
the yolk sac cells is a layer of mesoderm,
composed of occasional fibroblasts and
numerous large capillaries. Although these
embryonic vessels have thin walls, they
have no fenestrated areas.
The endodermal cells tend to be more
Choriovitelline placenta
cuboidal than in the bilaminar, omphaloIn sections from Bouin-fixed, paraffin- pleure. They have more numerous mitoembedded material, the choriovitelline pla- chondria than do the trophoblast cells and
centa appears to be as loosely applied to more extensive cisternae of the endoplasthe uterine epithelium as does the bilam- mic reticulum, with both granular eleinar omphalopleure. However, in glutaral- ments and numerous tubular agranular
dehyde- and osmium-fixed, plastic-em- elements (fig. 12). Between the borders of
bedded material, it is usually in close adjacent endodermal cells are extensive
apposition to the uterine epithelium (see dilated regions with numerous microvilli
inserts in figs. 7, 8). The trophoblast cells projecting into them. Small aggregates of
of the choriovitelline placenta are not as electron-dense particles are often found
thick as those in the bilaminar omphalo- apparently lying in the basement mempleure and do not contain as much in- brane and extracellular space beneath the
cluded material. Although the cell borders endodermal cells (fig. 12). A positive
cannot be discerned with the light micro- Perl's reaction for ferric iron was seen in
scope, they are readily apparent in elec- this region. Occasional granules with
tron micrographs. The microvilli of these similar appearance are found within the
trophoblast cells are long and numerous, endoderm itself.
but occasional stretches of surface lack
Marginal attachment areas
microvilli entirely. Extracellular material
is usually abundant between the microAt the periphery of each disc of choriovilli, and even where the trophoblast cells vitelline placenta is a modified region (figs.
are closely apposed to the maternal epi- 5, 6). Here the trophoblast cells become
thelial cells there is no interdigitation of markedly columnar. Where a band of these
apposed microvilli (fig. 7). At the base of columnar cells overlies a fold of endothe microvilli are found rough-surfaced mi- metrium they form a cap centered over the
cropinocytotic invaginations, and coated tip of the fold. From the middle of the
vesicles of various shapes are present be- capping group of trophoblast cells numerneath the surface. In both the thin and ous giant cells penetrate the uterine epithicker regions of the trophoblast mito- thelium and extend a short distance into
chondria are scarce and, although crystal- the endometrial stroma. The regions of
loid inclusions are not as abundant as in enlargement of the trophoblast cells conthe bilaminar omphalopleure, they still stitute an apparently continuous band
occupy considerable space in the cyto- around the choriovitelline placenta, but the
plasm. The Golgi membranes are not areas of penetration are discontinuous.
markedly juxtanuclear but generally oc- The columnlar trophoblast cells have many
mitochondria localized at the bases of the
cells, and the cytoplasm of these cells contains numerous polyrjbosomes. The giant
cells which extend into the endometrium
have several nuclei and a relatively emptyappearing cytoplasm. Where these cells
contact uterine epithelial cells, the two cell
types are closely appljed without intervening extracellular coats. In addition to areas
of interdigitation, they occasionally have
desmosomal junctions (fig. 11). The giant
cell cytoplasm extends down to the connective tissue and occasionally to the basement membrane surrounding the maternal
blood vessels, but has not been found to
penetrate the latter structure (fig. 10).
Unlike the regions where the trophoblast
is in contact with the uterine epithelium,
there is usually a basement membrane associated with the surface of the trophoblast where it penetrates into stroma.
Although the connective tissue of most of
the endometrium is edematous, at the tip
of the fold where the epithelium is penetrated are several slightly enlarged fibroblasts which may constitute a primitive
decidual reaction. In the term placenta,
the trophoblast cells in these areas are hyaline and appear to he degenerating.
The sinus terminalis projects into the
cavity of the yolk sac (figs. 1, 5), and is
generally covered by a layer of large endodermal cells which, except for their unusual size, are similar in structure to the
endodermal cells covering the rest of the
choriovitelline placenta.
As in the Virginia opossum, the endometrium is highly glandular, and extracellular substance accumulates not only
in the glands but also between the trophoblast and the luminal epithelium. The
uterine luminal epithelial cells are columnar and frequently contain both lipid
and glycogen, as well as multivesicular
bodies. The lateral and basal margins of
these cells contain numerous infoldings
that tend to be dilated as far up as the
level just beneath the apical junctional
complex (fig. 8). The apical microvilli
are numerous, are uniform in both length
and diameter, and cclntain a filamentous
core which extends into the cytoplasm
for a distance equal to or surpassing the
length of the microvillus (fig. 7). A distinct surface coat is present on the apical
surface of these cells. Numerous coated
vesicles are found between the bases of
these microvilli. The large Golgi complexes are supranuclear.
The uterine gland cells differ from the
luminal epithelial cells in having more
extensively developed cisternae of the
endoplasmic reticulum, often with numerous concenti-ically arranged cisternae. In
addition, the apical cytoplasm of these
cells contains large numbers of vacuoles
filled with flocculated material. The microvilli of the gland cells have very little
surface coat and do not have a central
bundle of filaments. Some of both the
luminal epithelial cells and gland cells are
ciliated .
The endodermal cells constituting the
epithelium of the allantois lack any extensive development of organelles that might
be associated with such specific activities
as protein uptake or secretion (fig. 13).
However an abundance of polyribosomes
would suggest that active protein synthesis
is taking place. The intercellular spaces
between these endodermal cells are dilated.
The vessels associated with the allantois
are evenly distributed and, although thinwalled, are large, rounded, and not in
particularly close association with the overlying epithelium.
Serosal chorion
It could not be determined whether
there was any region of the trophoblast
free of endodermal investment (serosal
chorion) at the stages examined. The yolk
sac around the vitelline arteries and
omphalomesenteric veins was highly convoluted and may have actually fused with
the mesoderm of the serosal chorion or
with itself, obliterating this structure. Because of the tortuous nature of the uterine
surf ace, careful dissection would probably
be better than serial sections for determining this point.
Several of the features of the placenta
of the four-eyed opossum can be thought of
as further development of the type of
placenta found in Didelphis. In Didelphis
the apposed areas of bilaminar omphalopleure of the adjacent fetuses fuse only
late in pregnancy, and presumably do not
rupture until parturition, whereas in
Philander there was only a remnant of
fused bilaminar omphalopleure left between two of the fetuses, and it did not
separate these fetuses. Furthermore, it is
stated by McCrady ('38) that in Didelphis
the embryonic membranes can be separated from the uterus without tearing at
all stages of pregnancy, whereas there is
a ring where the trophoblast penetrates the
uterine epithelium in Philander.
These features, plus the large size of
the vascular allantois, can be thought of
as evolutionary advances over the type of
placentation in Didelphis. An alternative
hypothesis is that the differences observed
represent a continuation of the normal
growth processes in a placenta that is retained for a longer period of time than
that of Didelphis. The 13 day gestation
period in Didelphis is the shortest known
gestation period of the marsupials, However, the gestation period of Philander is
unknown, and although it is quite possible
that it is longer, the fetal size at birth (11
mm) is comparable in the two species.
The arrangement of the areas of adhesion suggests at least two possible functions for this zone. It could function in
keeping the choriovitelline placenta closely
apposed to the endometrium, thus f acilitating gas exchange. It could also serve to
hold the placental membranes of the individual fetuses in a relatively k e d position after rupture of the fused bilaminar
It is particularly interesting that a mildly
invasive trophoblast, which closely resembles some of the features of more actively
invasive trophoblast often seen in the
chorioallantoic placenta, is developed in
the choriovitelline placenta of Philander.
The endometrial epithelium not only tolerates the invasion of this trophoblast, but
also forms adhesion areas and desmosomes
with it. This feature has recently been
found to be common in implantation
stages of several mammals that develop
chorioallantloic placentas (Enders and
Schlafke, '69).
As was expected, the trophoblast of
Philander shows features suggestive of
absorptive activity, including both coated
vesicles and multiple inclusions. Although
more included material is seen in the
trophoblast of the bilaminar omphalopleure, it seems likely that this accumulation of material is due to a less rapid rate
of turnover than in the vascularized portion of the placenta. Interestingly, the
yolk sac enldoderm does not show much
evidence of absorptive activity. Its cytological features are more suggestive of
synthetic activity. In this respect it resembles the yolk sac endodermal cells of the
little brown bat, Myotis lucifugus lucifugus
(Wimsatt and Enders, in preparation; see
also Wimsatt, '45) more than it does the
cytological descriptions of the absorptive
yolk sacs of the rat (Lambson, '66; Padykula, Deren and Wilson, '66) and rabbit
(Deren, Padykula and Wilson, '66).
The uterine luminal epithelium has the
type of microvillous border normally associated with rapid absorptive activity [a
feature often seen in uteri (Enders, '67)l.
The absorptive activity of the luminal epithelium may be important in the reabsorption of excess glandular secretion, in removal of the more fluid constituents of this
secretion, oir in the absorption of the products of breakdown produced by trophoblastic digestion, as well as in the more
obvious role of transport of materials from
the fetal membranes to the mother. It
would be necessary to have four-eyed
opossums available for experimental study
in order to discriminate among these and
other possible roles of the epithelium.
The rich vasculature of the allantois is
in striking contrast with the inactive appearance o€ its epithelium. By analogy
with gall bladder epithelium (Kaye,
Wheeler, Whitlock and Lane, '66) a weak
case could be made for fluid transport as
a function of the epithelium, in which situ.
ation the rich vascularization might be associated with the transport of this reabsorbed fluid. However, even assuming
some fluid absorption, the vascularization
appears out of proportion to the significance of this function.
Biggers, J. D. 1966 Reproduction in male marsupials. In: Comparative Biology of Reproduction in Mammals. I. 'W. Rowlands, ed. Academic Press, New York, pp. 251-280.
1967 Notes on reproduction of the
woolly opossum (CalurimtYs krbianus) in Nicaragua. J. Mammal., 48: 678-680.
Deren, J. J., H. A. Padykula and T. H. Wilson
1966 Development of structure and function
in the mammalian yolk sac. 11. Vitamin BIP
uptake by rabbit yolk sacs. Devel. Biol., 13:
- - - - - ..
Enders, A. C. 1967 The uterus in delayed jmplantation. In: Cellu1a:c Biology of the Uterus.
k. Wynn, ed. Appletorr Cent;;y Crofts, New
York, pp. 151-190.
Enders, A. C., and S. Schlafke 1969 Cytological aspects of trophotilast-uterine interaction
in early implantation. Am. J. Anat., 125: 1-30.
Enders, R. K. 1935 Mammalian life histories
from Bano Colorado Island, Panama. Bull.
Mus. Comp. Zool., Harvard, 78: 385-502.
1966 Attachment, nursing and survival
of young in some didelphids. In: Comparative
Biology of Reproduction in Mammals. I. W.
Rowlands, ed. Academic Press, New York, pp.
Hall, E. R., and K. R. Kelson 1959 The Mammals of North America. Ronald Press, New
York, p. 10.
Kave, G. I.. H. 0. Wheeler, R. T. Whitlock and
N.-Lane' 1966 Fluid transport in the rabbit
gallbladder. J* Cell BiOl., 30: 237-268.
Lambson, R. 0. 1966 A n electron microscopic
visualization of transport across rat visceral
yolk sac. Am. J. Anat., 118: 21-32.
McCradv. E. 1938 The embrvologv
-_ of the
opossum. Am. Anat. Mem., 16: 5-233.
Padykula, H. A,, J. J. Deren and T. H. Wilson
1966 Development of structure and function
in the mammalian yolk sac. I. Development
morphology and vitamin BIZ uptake of the rat
yolk sac. Devel. Biol., 13: 311448.
Phillips, C. J., and J. Knox Jones, Jr. 1968
Additional comments on reproduction in the
woolly opossum ( CaZuromys derbianus) i n
Nicaragua. J. Mammal., 49: 320-321.
Romer, A. S. 1966 Vertebrate Paleontology.
University of Chicago Press, Chicago.
Sharman, G. B. 1961 The embryonic membranes and placentation in five genera of diprotodont marsupials. Roc. Zool. SOC.Lond., 137:
Simpson, G. G. 1945 The principles of classification and a classification of mammals. Bull.
Am. Mus. Nat. Hist., 85: 1-350.
Wimsatt, W. A. 1945 The placentation of a
vespertilionid bat, Myotis Zucifugus lucifugus.
Am. J. Anat., 77: 1-51.
Left uterus of the four-eyed opossum, including ovary and left vaginal
canal. A small portion of the bladder shows at the bottom of the
plate. This horn contained five fetuses. X 2.5.
3 Hemisected uterus showing two of the fetuses. The allantois of the
fetus seen in dorsal view has been cut open. X 3.
Section of the uterus in figure 3. The allantois is nearly as large as
the embryo. The fetus surrounded by amnion (barely visible at arrow) is situated in a common yolk sac. A sinus terminalis (ST) at
the top of the picture marks the margin of the vascular yolk sac of the
fetus sectioned. X 16.
Allen C. Enders and Robert K. Enders
Section through the margin of the vascular yolk sac placenta. The
portion of the yolk sac above the sinus terminalis (ST) is vascular
(VYS) and is consequently part of the choriovitelline placenta; the
portion below it is avascular (AYS) and is consequently part of the
bilaminar omphalopleure. Between the two ~xrrowsthe trophoblast
cells have penetrated the tip of a n endometrial fold. The residual
shell membrane is seen enclosed by a fold of bilaminar omphalopleure, and a t the upper left corner the allantoic sac is seen enclosed
by a layer of yolk sac endoderm (YS). Note that here where the
endoderm of the yolk sac is separated from the allantois, it is clearly
the allantois that is vascularized. X 50.
Region of penetration of the uterine epithelium by trophoblast (TI).
The trophoblast which constitutes the outer layer of the choriovitelline
placenta becomes columnar in this region, and trophoblastic giant
cells penetrate the tip of the endometrial fold. Several small vessels
underlie the yolk sac endoderm (YS). A portion of the allantois is
seen a t the left. Note the contrast between the epithelium of the
gland ( G ) and the luminal epithelium of the endometrium. Fibroblasts are more numerous i n the stroma a t th'e tips of the folds than
they are deeper i n the endometrium. X 335.
Allen C. Enders and Robert K. Enders
Electron micrograph of trophoblast from the choriovitelline placenta.
Note that although the microvilli of the trophoblast are long, they
do not interdigitate with the microvilli of the uterine epithelial cell
above. The arrows in the uterine epithelial cell point to filaments
which pass into the cytoplasm from the microvilli. There are basal
infoldings ( B F ) into which microvilli project on the basal surface of
the trophohlast. The basal lamina (BL) does not enter these folds.
The insert at the lower right shows a plastic section of this material.
The region in the electron micrograph is similar to that depicted by
the rectangle i n the insert. X 24,700. Insert x 420.
Allen C. Enders and Robert K. Enders
Uterine luminal epithelium. Note the intercellular space stretching
from beneath the junctional complex (JC) down to the region of basal
infolding of the cell membranes. The insert depicts a region similar
to that shown in the electron micrograph, where the choriovitelline
placenta is overlying the luminal epithelium. No difference was seen
in uterine epithelial cells adjacent to bilaminar omphalopleure.
x 10,400. Insert X 420.
Allen C. Enders and Robert K. Enders
Trophoblast from the bilaminar omphalopleure. The uterine surf ace
is a t the upper left, the basal surface at the lower right. Short sections of cisternae of the endoplasmic reticulum and mitochondria
with lamelliform cristae are distributed throughout the cytoplasm,
and a small cluster of Golgi membranes is present in the center of
the micrograph. The most characteristic feature of these cells, however, is the complex inclusion bodies ( I B ) which occupy a major
portion of the cell and contain crystalloids and other formed inclusions. X 16,000.
10 A uterine vessel overlain by trophoblast. The rather empty cytoplasm
of the trophoblastic giant cells penetrates the endometrium as f a r as
the uterine vessels, but has not been seen to penetrate their basal
laminas. x 10,000.
Trophoblast cell (above) sharing a desmosome (arrow) with a uterine
epithelial cell (below). X 21,000.
Allen C. Enders and Robert K. Enders
12 Apical end of a n endodermal cell from the choriovitelline placenta.
Note the dilation of the intercellular space at the right and the
abundance of both mitochondria and endoplasmic reticulum. The
insert shows a series of laminar granules usuarlly found between the
yolk sac and underlying fibroblasts or endothelial cells. A positive
Perl's reaction for ferric iron can be demonslrated here i n paraffin
embedded material. x 16,500. Insert x 18,100.
13 Endodermal cell from the allantois. Note the, relative simplicity of
this cell compared to the yolk sac endoderm cell above and also the
dilation of the intercellular space. x 11,000.
Allen C. Enders and Robert K. Enders
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