Paper - The role of the primitive mesothelium in the development of the mammalian spleen: Difference between revisions

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#REDIRECT [[Paper - The role of the primitive mesothelium in the development of the mammalian spleen (1936)]]
 
Edvvard Agustus Holyoke
 
 
Department of Anatomy, College of Medicine, University of Nebraska, Omaha,
Nebraska
 
TWO PLATES (rwrmvn mavens)
 
It is generally accepted that the embryonic mammalian
spleen can first be recognized as a dense accumulation of cells
in the mesenchyme of the dorsal mesogastrium or neighboring
rnesentery; The splenie rudiment has been described by
Kolliker (1854), Miiller (1870), Minot (1892), Kollmann (’O0),
Tonkoff (’0O), Sabin (’12), Thiel and Downey (’21) and many
others. Choronsehitzky (’OO) and Danchakoff (’16) have observed a similar anlage in birds. Radford ( ’-08) believed the
splenic anlage’ of the frog to be a dense accumulation of
lymphocytic cells around the mesenteric artery, While Leon
(’32) placed it (in fish) at a definitely predetermined point in
the vascular endothelium of the sub—intestinaI vein.
 
The old controversy concerning the germ layers involved in
the developing spleen has apparently been settled, the
generally accepted view today being that it is entirely of ‘mesodermal origin. However there still exists some doubt as to
which mesodermal elements are directly involved. In a recent paper, Bergel and Gut (’34) have ‘attempted to establish
 
the human spleen as a derivative of mesenchyme alone as has i
 
been held for birds and reptiles by Danchakoff (’16) and
others, and for Amphibia by Nakajima (’29). On the other
hand Kollmann (’OO), Tonkofi (’00), Choronschitzky (’00),
Thiel and Downey (’21) and I-Iartmann (’30) have shown that
the spleniclrudiment receives cells from the overlying peri
toneal layers which are crowded into the mesenchyme during the early stages of development. Toldt (1889), and Janosik
( ’01) were of the opinion that the spleen was exclusively mesothelial in origin.
 
Recently I have had occasion to examine the splenic region
of an extensive series of early human‘ and pig embryos sectioned in various planes and stained by several different
methods. Observations of this material have led me to conclusions differing from those of Bergel and Gut.
 
 
Inasmuch as most descriptions of the mesenteries before
the splenic rudiment appears have been very incomplete, a
careful examination of very early material was made. As
an exception to such incomplete descriptions stands the work
of Thiel and Downey (’21) ; these authors described the dorsal
mesogastrium of a 3—mm. gopher embryo, in which stage they
found a definite mesentery covered by a loose visceral peritoneum and containing a few formed capillaries. These
authors have also described the dorsal mesogastrium of the
pig embryo (7.5 mm.) before the appearance of the spleen.
 
 
In human embryos of 3 mm? I have found no indication
whatever of the future splenic rudiment. The region in which
the rudiment is to appear can, however, be roughly determined
by its relation to the stomach. Examination of the mesentery
in that region reveals no characteristic findings. Like all
other parts of the mesentery it consists of a comparatively
loose mass of mesenchyme covered on its lateral surfaces by
dense mantles of irregularly arranged cells which are three or
four layers deep. This coelomic epithelium or primitive mesothelium is in a state of active proliferation as evidenced by
many mitotic figures. The cells composing these layers bear
a striking resemblance to those of the underlying mesenchyme
 
’ Of the human embryos examined some were from the collection of the Depart~
ment of Histology and Embryology, Cornell University Medical College, Ithaca;
some from the collection of the Department of Anatomy, Harvard Medical School;
and the remainder from the collection of the Department of Anatomy, University
of Nebraska Medical College. The author is most appreciative of the generosity
of Dr. B. F. Kingsbury and Dr. J. L. Bremer, who permitted the examination of
this material and the use of some of the figures herewith reproduced. Pig
embryos examined were locally collected and prepared by the author.
 
“All measurements of embryos given in this paper refer to crown-rump length.
 
 
 
in that they take an identical cytoplasmic stain and their nuclei
are large, containing a few dense masses of chromatin and
usually two nucleoli. They are intimately connected with the
mesenchyme, protoplasmic continuity being demonstrable in
many places. The possibility of .many cells passing from coelomic epithelium to mesenchyme can be readily shown by the
intimate contact and irregular boundary between the surface
layers and underlying tissue, and by the presence of mitotic
spindles lying perpendicular to the surface of the mesentery
as described by Thiel and Downey for the 7 .5—mm. pig embryo.
 
 
Essentially the same conditions have been observed in other
human embryos up to 6 mm. in length. By the time this stage
has been reached, the stomach has become a large conspicuous
organ and has started its rotation toward the left. In -rotating
it has carried a fold of the mesogastrium with it so that, as
seen in transverse section, it makes one acute bend at its root
where it joins the body wall and another at its stomach attachment (fig. 1). Near the midpoint of this mesenteric segment there has developed a distinct bulging toward the left.
 
 
This bulging of the mesentery is due to a proliferation of
mesenchymal cells in that region, but there is no demonstrable
condensation. The coelomic epithelium there has retained all
of the characteristics described for the earlier embryos. It
has remained an irregularly formed mantle of cells continuous
With the underlying mesenchyme, and as indicated in the
earlier forms, presenting no cytological characteristics which
would enable one to distinguish between the two. The only
variation of any kind is a slightly deeper’ cytoplasmic stain
in the surface cells due, possibly, to crowding.‘
 
 
Elsewhere the general coelomic epithelium has started to
differentiate. In most regions it has become a cuboidal or
squamous layer sharply cut off from the mesenchyme by a
distinct basement. membrane. Examples of this are to be
seen in figure 1. It will be noted that the coelomic epithelium
on the right side of the mesogastrium has become cuboidal,
but that the basement membrane is not yet demonstrable.
There remain, however, a few other areas where the coelomic epithelium has retained its earlier form, notably in a series of
irregularly located points along the gut. These same conditions are to be found in pig embryos of 7.5 to 8 mm.
 
The above described bulging of the mesentery probably
should not be considered as the splenic anlage for a definite,
much less extensive condensation of cells soon appears in this
region. In 10- to 12-mm. pig embryos this mass is present
in the left part of the mesentery Where it is continuous with
both the primitive mesothelium and the mesenchyme (fig. 2).
It does not stand out very conspicuously at this stage because
of the density of the mesenchyme in general. The cellular
mass is not in contact with the mesothelium of the right leaf
of the mesentery, but is separated from it by a small area of
looser mesenchyme.
 
 
A closer study of the region of the anlage in the 10—mm. pig
embryo (fig. 2) fails to reveal any line of demarcation between
mesothelium and dense mesenchyme. As indicated in the
earlier stages of pig and human embryos continuity between
the two can be demonstrated and no cytological differences,
except as noted, are to be found to separate them. Mitoses
are abundant and these occur alike in the deeper cells of the
anlage and the free surface of the mesentery A few of the
latter figures still present spindles perpendicular to the surface indicating a contribution of cells from this region to the
deeper mass. All of these cells have a very similar morphology. The deeper mesenchymal cells are so densely
packed that their intercellular bridges are difficult or impossible to distinguish as separate cytoplasmic strands, and
their typical stellate form is obscured. These cells, as Well as
those near the surface, take a deep basic cytoplasmic stain
against which the vesicular nuclei stand out as comparatively
pale bodies.. The nuclei are still characterized by a few dense
chromatin masses and two nucleoli, as in the earlier embryos.
The less dense mesenchyme at the right of the anlage shows
nuclei identical to those just described. The cytoplasm of
these cells stains less deeply and the typical stellate form is
more readily demonstrable. Similar conditions obtain in
human embryos of from 8 to 9 mm. in length.
 
 
From these findings one is forced to two conclusions. First,
there is no morphological basis for believing that the primitive
mesothelium is more than a. dense layer of littoral mesenchymal cells which later differentiate to form tl1e adult peritoneal epithelium. Second, both the surface and deeper layers
play an important role in the contribution of cells to the
splenic mass.
 
It has been repeatedly shown that primitive mesothelial cells
are pleuropotential and that invasion of the deeper tissues by
them is of frequent occurrence (Schott, ’O9; Mollier, ’O9;
Bremer, ’14; Ilaff, ’14; Scammon, ’15; and others). Emmel
(’16) thought that many of the free cellular elements found in
the peritoneal cavityof the pig embryo were derived from
this same source.
 
The time of appearance of a definite splenic anlage seems
to be relatively constant in many mammalian species. I have
found it in human embryos at 7.5 to 8 mm. Tonkoff (’00),
Kollmann ( ’OO), Sabin (’12), Ono (’30) and others have described the splenic rudiment in embryos of from 8 to 10 mm.
or during the fifth week of development, while Bergel and Gut
(’34) found it at 7 mm. In the pig embryo the time of appearance of the spleen may be a little more variable. In some of
the series of embryos I have examined, it was poorly developed at 12 mm., while some 10—mm. series showed it distinctly. It is possible that slight errors in measurement and
failure to make adequate allowance for shrinkage during
preparation may account for this variation. Thiel and
Downey (’21) described a definite splenic anlage in the pig
embryo at 15 mm., and a well-developed splenic rudiment in
the striped gopher at 8 mm. In an 8-mm. cat embryo which I
examined, it could be identified clearly.
 
After the splenie mass has been established, it grows very
rapidly causing a more and more marked local bulging of the
mesentery which gradually replaces the more diffuse enlargement of the earlier presplenic stages. At the same time the
mesentery begins to buckle toward the left at the splenic
prominence so that this structure soon lies in the convexity of a definite fold (figs. 4, 5, 7 and 9). Coincidentally, the
coelomic epithelium begins to become separated from the
splenic anlage as a distinct layer. This is first indicated by
the organization of the cells into a single layer and the appearance of a distinct basal zone, free from nuclei and conspicuous
because of its light color. Ono (’30) has mentioned this zone
in the early human embryo. In my opinion this light zone
owes its appearanc solely to segregation of the nuclei in more
peripheral portions of the superficial cells. This leads me to
the conclusion that the plane of sectioning is not oblique
enough in relation to the epithelial surface seriously to distort
the picture. These changes just noted are usually demonstrable in 12-mm. pig embryos (fig. 3), although at many
points the cellular arrangement is still irregular. and protoplasmic continuity with the mesenchyme is evident. At a, b,
and c are shown small groups of cells continuous with the
coelomic epithelium, interrupting the light zone and apparently making their way into the splenic mesenchyme. In a
few places in this surface layer there are signs of nuclear
differentiation, the chromatin breaking up into finer particles
than in the mesenchymal nuclei. Ad basement membrane soon
appears in several places and begins to extend along the deep
surface of the light zone. Between the Various segments of
this membrane, protoplasmic continuity with the mesenchyme
remains and the migration of superficial cells apparently continues. The organization _of epithelial cells is complete in
human embryos of from 10 to 12 mm. (fig. 4). These cells
become more distinctly separated from each other by cell membranes a11d the general nuclear stain becomes more intense. In pig embryos of 14 to 15 mm. a similar series of
changes begins. In a 14-mm. pig embryo the organization of
the epithelium into a distinct layer’ is almost complete and the
light zone is conspicuous. The basement membrane has not
appeared except in a few isolated places and cells are still
passing into the underlying mesenchyme (fig. 8).
 
Soon after the organization and dfferentiation of the
coelomic epithelium has begun the basement membrane is almost complete, cutting off a distinct layer of columnar cells.
When the basement membrane appears, it is readily demonstrable by ordinary staining methods. In a typical human
embryo of 13.5 mm. the membrane stands out conspicuously
as a sharp line broken only in a few places by migrating cells
(fig. 5). One of these regions is shown in more detail in
figure 6. The continuity of the coelomic epithelium with the
splenic mesenchyme has remained unbroken at this point, and
a cell (mc.) is being added to the splenic anlage. There
probably is only an occasional cell added to the mesenchyme in
this manner subsequently, for (as can be seen in figs. 5 and 6) the mesothelium is now a distinct layer and is sharply demarcated in most places. A closely graded series of nuclei can
now be found in this mesothelial layer; representing various
steps between the primitive mesothelium, which is identical
with the mesenchyme, and the general mesothelium of this
stage of development. A few of these nuclei retain mesenchymal characteristics (fig. 6): Others show a finer distribution of chromatin and the disappearance of one or both
nucleoli; and still others are intensely stained and contain
finely divided chromatin, in some cases appearing as a
delicate network. These are typical of the general mesothelium. There has also been a progressive limitation of the
more primitive type of mesothelium to the splenic region
alone. In early human embryos (figs. 1 and 4) this primitive
non-specific layer extends beyond the locus of the splenic
anlage over the surface of the stomach. Going beyond this
region in 6- to 7-mm. human embryos, one sees a gradual and
continuous transition to the general coelomic epithelium
(fig. 1). Later (fig. 5) this transition begins definitely at the
margin of the spleen. The cells become progressively more
and more flattened as the layer is traced away from the spleen,
the light zone narrower and the nuclear stain more intense.
The general coelornic epithelium is a low cuboidal or squamous
layer and somewhat resembles the adult condition.
 
During the same period the splenic mass becomes more distinct from the surrounding mesentery, the transition from dense to loose mesenchyme becoming more abrupt and the
mesenchyme of the mesentery more attenuated. At 11 mm.
(fig. 4), the human spleen is not Very clearly marked off, but
at 13.5 mm. (fig. 5) the limits of the anlage are very distinct.
Similar changes have appeared in the splenic anlage of the
14-mm. pig embryo (fig. 7). The dense mesenchyme here is
beginning to show sharp localization. It can be distinctly
differentiated from the loose mesenchyme to the right but is
not so definitely demarcated on all sides, tending to merge
more gradually into the looser surrounding tissue.
 
As the spleen continues to grow, it expands in the left half of
the mesentery and grows around the loose mesenchyme at
the right (fig. 9). The large splenic vessels are found here in
the concavity of the splenic fold (figs. 4, 5 and 7) and this
region becomes established as the hilus. This arrangement is
definitely shown by human embryos of from 15 to 16 mm.
 
From this time on the coelomic epithelium over the spleen
diflerentiates rapidly. The light zone becomes narrower as
the cells begin to assume a low columnar or cuboidal form, and
the basement membrane becomes complete (fig. 9). There remain a few irregularities, in the deep surface of the mesothelium, but it is doubtful whether they could be interpreted
as indicating it further exchange of cells. This process is
probably complete in human embryos of 14 mm. and in pig
embryos of from 15 to 19 mm. The coelomic epithelium of a
22-mm. pig embryo is composed of cells regularly placed upon
a definite basement membrane which are sharply separated
from one another by distinct cell membranes. By this time
they difier from the general mesothelium only in that they are
cuboidal rather than squamous in form. This latter condition
is not attained in the splenic region of pig embryos before they
reach a length of 45 to 50 mm. Figure 11 shows a cuboidal
layer over the spleen at 35 mm. In man the adult type of mesothelium appears at a much earlier stage. The mesothelial layer over the spleen at 22 mm. is identical in every way with.
the general coelomic epithelium and is approaching the adult
condition.
 
 
Bergel and Grut (’34) have recently denied any contribution
by the eoelomic epithelium to the underlying mesenchymal
anlage of the mammalian spleen. Nakajima (’29) ha.s come
to similar conclusions in a study of amphibian embryos.
From Nakajima’s discussion and figures, I am unable to determine how these conclusions were reached. Bergel and Gut
(’34) made an extensive study of human material. In their
opinion most investigators have been misled on the mesothelial question, because they have failed to use specific stains,
and have studied sections which have not cut the epithelial
layers transversely. They state that the basement membrane
between the coelomic epithelium and the underlying mesenchyme is demonstrated only by strong cytoplasmic stains, and
that the usual technical methods fail to show it. With this
statement in mind, I have reexamined all of my material,
some of which has been stained with M-al1ory’s connective
tissue stain and some by B.ielschowsky’s silver impregnation
method. I can find no instance in which these preparations
show a complete basement membrane between the eoelomic
epithelium and the splenic anlage in pig embryos of less than
15 mm. On the other hand very distinct basement membranes
are demonstrable in relation to the general eoelomic epithelium
in preparations stained with Delafield’s hematoxylin and
azure II eosin, or with Wright’s and Giemsa stains. All of
the material used for illustrations of this paper (except fig. 9,
where orange G was used with hematoxylin) has been stained
by these or similar methods. Basement membranes are conspicuously demonstrated at some point in most of these figures.
Figure 1, page 24 of Bergel and Gut ’s paper, they considered
a demonstration of a complete basement membrane in the
splenic region of an early human embryo (14 mm.). In my
opinion this membrane is no more distinct than that shown by
other methods in my material. Furthermore the leader
pointing out the membrane in their figure could be interpreted as indicating a typical break with the continuity between
superficial and underlying cells.
 
 
In regard to the plane of sectioning of the material, there
is no doubt that misleading impressions can be given by
sections which are not truly transverse through the layers in
question. However, the condition of the coclomic epithelium
here represented (except where specially noted) is not to be
explained on this basis. Regardless of the plane of sectioning,
the condition of the mesothelium over the splenic region (figs.
1 to 9) is a constant finding in these early embryos. On the
hilar side of the dorsal mesogastrium in the same sections,
the mesothelium is a cuboidal or squamous layer. Further
evidence that these sections are nearly transverse was ob
tained by tracing the layer through several sections and ,
 
demonstrating that it shifts only slightly in relation to other
structures, surely no more than the comparatively fiat layer
on the opposite side of the mesentery. In those stages where
several layers of primitive mesothelial cells are found, care
was taken to demonstrate that similar conditions were not
present on the opposing surface. Also, there are other findings such as cytoplasmic continuity between layers, the
presence of transitional cell types and mitotic spindles perpendicular to the surface of the mesentery, which are not to be
explained on the basis of plane of section.
 
Bergel and Gut agree with Hartmann ( ’30) that cords of
epithelial cells can be seen growing down into the rudiment.
They maintain, however, that these cells are surrounded by a
definite basement membrane andlare not incorporated in the
mesenchyme. In their figures. one could not definitely determine such a membrane.
Finally these two authors state that, because two types of
cells are not demonstrable in the splenie anlage, this structure could have been derived from one source only. While it
is true that Toldt (1889), Janosik (’01) and others have
thought that the free cellular elements of the mammalian
spleen were cut off from the coclomic epithelium and entered
_a mesenehymal meshwork, and more recently Radford ( ’O8),
Leon (’32), and others have found free cellular elements at
early stages in lower forms, Thiel and Downey (’21), Danehakoff (’16) and others have shown than in anmiotes the
splenic rudiment contains no free cells until a later time, and
that such cells are cut off from the mesenchymal syncytium
when they do form. This is in accord with my own observations. I am aware of no contention made by recent authors
that the coelomic epithelium contributes free elements or any
differentiated forms whatever to the spleen. The evidence
rather points toward the continuity of mesothelium and mesenchyme. The cells which are crowded into the latter layer are
originally a part of the syncytium and remain so. 'In conclusion I should like to point out that the contribution
of cells from the coelomic epithelium is important more because it indicates that this layer is essentially mesenchymal
in its nature and potentialities than because it indicates a
dual source of the splenic rudiment. As the coelomic epithelium is becoming a differentiated layer, this contribution continues sparingly for a time; but. after a certain critical
point is reached, it ceases entirely.
 
 
 
SUMMARY
 
The splenic rudiment appears in the human embryo at 7.5
mm. and in the pig embryo at 10 mm. It is first observed in
other mammalian forms at comparable stages.
 
The anlage develops as a dense mass of mesenchyme in a
preformed bulging of the dorsal mesogastrium. It is at first
continuous with the overlying coelomic epithelium and the
mesenchyme of the mesentery. It gradually becomes more
sharply delineated from this latter tissue, which becomes more
diffuse as the anlage becomes more condensed.
 
A small area of loose mesenchyme always separates the
anlage from the right leaf of the mesentery. The splenic
vessels appear in this area and the spleen grows around it
forming the hilus at this point.
 
Before the splenic rudiment appears the coelomic epithelium
is present as a surface condensation of mesenchyme continuous with the underlying tissue.
 
 
The spleen forms at the base of the littoral condensation to
the left of the dorsal mesogastrium and is derived in part
from this and in part from the underlying mesenchyme. The
surface layer over the spleen retains its primitive character
for a much longer period than coelomic epithelium in general.
 
Differentiation of the primitive coelomic epithelium in contact With the splenic rudiment can first be recognized by an
organization of the component cells into a simple cuboidal
layer. This layer later becomes sharply cut off from the
underlying tissue by a definite basement membrane. Until
this membrane becomes complete some of the cells retain their
primitive potentialities as is evidenced by their mesenchymal
type of nucleus and by their occasional migration into the
splenic mesenchyme.
 
After the basement membrane becomes complete, all migration ceases. These mesothclial cells then become progressively
flattened until the adult squamous form is approached. This
condition is attained at a much earlier stage in the human
embryo than in the pig.
 
The primitive mesothelium plays an important role inthe
development of the mammalian spleen. This role indicates
mesenchymal potentialities in the primitive mesothelium
rather than a dual source of the spleen.
 
 
 
LITERATURE CITED
 
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O.\'o, K. 1930 Untersuehungen iibcr die Entwi‘cklu11g der n1esch11'ehen Milz.
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Plates
 
 
1 The splenie region of a 6—mm. human enibryo showing the relations in the
dorsal mesogastrium before the spleen appears. There is a local bulging of the
mesentery but no condensation. The primitive ooelomic epithelium is a dense
mantle of cells continuous with the underlying meseneiiynial syneytiuin and with
the differeiitiaterl mesothelium at the root of the n1esentery_ Ilemaroxylin and
eosin. Photo X 200.
 
2 The splenic anlage of a 10—mm. pig leml)ry0 showing the dense local mass
of cells continuous and morphologically similar h0tl1 with the eoelomic epithelium
and the adjacent mesenehyme. Wright’s and Giemsa stain. Photo X 550.
 
3 The splenie anlage of a 12-mm. pig embryo. The cells are a.1'r2111g'e(l in :1
single layer and a ‘ light zone’ is apparent at some points between the peripheral
nuclear stratum and the nleseiiehyme. There is no basement membrane and the
cells at a, b and e are niigratirig into the mesenchyme. The apparent double
nuclear layer at some points inclioates that the section is not exactly transverse.
I)elafield’s hematoxylin and azure ll eosin. Photo X 750.
 
4 Human embryo, 11 mm, showing the early buckling of the dorsal meso,<2;ast1'ium toward the left and the appearance of the splenic vessels in the coneavity of the resulting fold. The splenie anlage is indieatecl by the prorninenee
at the eoxnexity of the fold as it does not stand out clearly from the adjacent
dense mesenchyme. The eoelomie epithelium has become a simple eolumnar layer,
but there is no basement membrane. There is a. well-defined euboidal. layer on
the concave surface of the mesenteric fold. llematoxylin and eosin. ‘Photo X 200.
 
5 Human embryo, 13.5 mm. The splenie anlage is a distinct condensation of
mesenchyme. The overlying eoelomio epithelium has become a low columnar
layer marked off by a. basement membrane except in a few places (fig. 6). This
columnar layer is continuous with the cuboidal layers beyond the margins of the
anlage. Hematoxylin and eosin. Photo X 200.
 
6 Detail from preceding figure. The (tell me. is being added to the splenio
Inesenchyme. nm. is a typical mesenchymal nueleus in one of the epithelial cells.
Photo X 400.
 
 
EXPLANATION on FIGURES
 
7 Pig embryo, 14 mm., showing the forination of the splenie fold in the dorsal
niesogastrium and the bulging due to the growth of the anlage. The splenie eondensation is distinct from the surrounding niesencliyiiio, particularly in the region
of the splenic Vessels.
 
8 Detail froin preceding figure, showing the euhoida.l eoelomie epitheliuin and
the early formation of the light zone. 4 and 1) indicate cells passing into the
inesenellyine. Photo X 550.
 
9 Huinan embryo, 15 mm. The spleen is situated in the convexity of :4, sharp
be-ml in the 1n(vse.11tory. The hilus is located in the loose iricsenehyme in the
eoiieavity of this bend. The eoelomie epithelium is a columnar layer completely
out off from the spleen by :1 basement nnenibrane. Hematoxylin and orange G.
Plmto X 200.
 
10 Pig embryo, 22 mm. There is 4, distinct euboidal layer of aoeloniic.
epithelium sharply mzu'ked off by :1 haseinent. membrane. Delafield’s henmt.ox_ylin
and azure II eosin. Photo X 550.
 
11 Pig embryo, 35 mm., showing the persistence of a euboidal type of mesotheliuin over the spleen. De1afie]d’s hematoxylin and azure II eosin. photo X 750.
 
12 Human embryo, 22 mm. There is a distinct low euhoidal layer of 1Yl(',8()thelium simulating somewhat. the adult, condition. Ilexrmtoxyliii mid eosin.
Photo X 750.
 
 
 
 
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[[Category:Spleen]][[Category:1930's]][[Category:Historic Embryology]]

Latest revision as of 11:37, 12 June 2020