The early development of the cat 3

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Hill JP. and Tribe M. The early development of the cat (Felis domestica). (1924) Quart. J. Microsc. Sci., 68: 513-602.

1924 Cat Development: 1. Ovum of the Cat | 2. Process of Cleavage | 3. Formation of the Blastocyst | 4. Discussion | Plates | cat

Chapter Iii.—Formation of the Blastocyst

1. Late Morula and Early Blastocyst Stages from the Uterus

The material available for the study of these stages, of especial interest for the elucidation of the mode of formation of the blastocyst, consists of (a) a series of five eggs from the uteri of Cat 5.1.10, labelled A-E, three from one uterus, two from the other, and ranging from the solid morula, through morulae in which the blastocyst-cavity is appearing to the early blastocyst, and (b) two uterine eggs from Cat 4.6.19. We have not found it possible to make an accurate count of the blastomeres in these eggs.

Morula 29 (5.1.10 E). Diameter, 0-128 x 0-10 mm. Diameter of morula, 008 x0-072 mm. Zona, 0-0096 mm. Pig. 22, PL 27.

The morula is ovoidal in section (fig. 22) and is separated by a space from the very thick zona. Such a space occurs in all the eggs of this series, though it is quite narrow in blastocyst 82. Whether it is the result of contraction during fixation or represents a fluid-filled space in the living egg, we are unable to determine. We have drawings of some of the morulae of this series, made with the aid of the camera lucida, whilst they were still in the fixative (picro-nitro-osrnic acid), in which the space is clearly shown, and in this connexion it is worthy of note that Assheton (3) states in the case of the Eabbit that ' up to the moment of the beginning of the [blastocyst] cavity . . . the embryo may often be found to be slightly retracted from the zona radiata in the fresh state ' (p. 131). The morula consists of an outer rather more deeply staining layer of trophoblast, more or less clearly marked off from the somewhat lighter-staining mass of central or embryonal cells. The trophoblast appears as a single layer of large cuboidal to more flattened cells with large nuclei and sparse fat-globules. The central mass fills the space inside the trophoblast and is composed of large polyhedral or polygonal cells, also with large nuclei and containing fat-globules. Between the cells, as also between them and the trophoblast, are well-marked intercellular spaces, some cleft-like, others irregular, no doubt filled with fluid in the living egg. There is already evidence of the commencing degeneration of some of the central cells. In the section following that represented in fig. 22, three such cells with small degenerating nuclei are met with immediately below the trophoblast. In one the cell-body is indefinite, the nucleus is about half the diameter of that of the normal central cell and has its chromatin massed on one side of the nuclear membrane ; in a second the cell-body is again ill-defined, and the nucleus is still smaller, darkly staining, and pycnotic ; in a third the small cell-body contains a correspondingly reduced bilobed and pycnotic nucleus.

One trophoblastic and one central cell are in process of division.

Morula 30 (5.1.10D). Diameter, 0-12 x 010 mm. Diameter of morula, 0-076 x 0069 mm. Zona, 0-012 mm. Figs. 23 and 24, PI. 27.

This morula is very similar to the preceding. The central cells, however, are rather more loosely arranged, the intercellular spaces being more marked, whilst there is unmistakable evidence of cytolysis of certain of the cells (fig. 24). On the right in the figure is to be seen a cell with a quite degenerate deeply staining nucleus, whilst towards the bottom left of the figure are two quite degenerate cells with shrivelled deeply staining nuclei. In the section preceding that represented in fig. 24 occurs a small pycnotic nucleus lying free in a space apparently formed by vacuolation of the cell-body. TAVO of the trophoblastic cells are in mitosis. Sparse fat-globules are present in the trophoblast, but they appear to have largely disappeared from the central cells.

Morula 81 (5.1.10C). Diameter, 0-12x0-11 mm. Diameter of morula, 0084x0072 mm. Zona, 0-012 mm. up to 0-014 mm. Fig. 25, PI. 27.

This morula generally resembles the preceding two, but on one side, between the trophoblast and the central cells, there is now present a continuous somewhat irregular space extending through six sections and containing remnants of a delicate cytoplasmic reticulum and, in fig. 25 towards the left, a small degenerate nucleus. This space is the beginning of the blastocyst-cavity. We regard it as being formed partly by the breaking down of certain of the central cells, partly by the flowing together of fluid-filled intercellular spaces. The central cells are separated here and there by irregular intercellular clefts, but on the whole are more compactly arranged than in morula 30. Fat-globules are present both in the trophoblast and the central cells, in the latter in fair numbers. One of the trophoblast cells is in mitosis.

Blastocyst 32 (5.1.10B). Diameter, 0-108x0-105 mm. Diameter of blastocyst, 0-079 x 0075 mm. Zona, 0-012 mm. Pig. 28, PI. 28.

In this egg the blastocyst-cavity is represented by two spaces of unequal size which extend up on one side of the blastocyst from the lower pole to just past the equator, the sectional plane being apparently transverse to the polar diameter. The spaces lie between the trophoblast and the mass of central cells (fig. 28), and appear empty except for the presence of traces of cytoplasmic detritus adjoining the central cells and, in the larger, of a very degenerate cell-remnant. The central cells are separated here and there by intercellular spaces and appear similar except that certain of them have more eosinophil cell-bodies than the others. There is no evidence of degeneration in the main body of the central cells.

Two central cells and one trophoblastic are in mitosis.

Blastocyst 33 (5.1.10 A). Diameter, 012x010 mm. Diameter of blastocyst, 0-08 x 0-067 mm. Zona variable up to 0-012 mm. Fig. 29, PI. 28.

The blastocoele is now a continuous cavity situated in the lower hemisphere of the blastocyst and measuring 0024 x 0043 x 0-048 mm. It is bounded^ below and at the sides by the trophoblast and above by t h e ' mass of central cells. The blastocyst stage is thus definitely established. It consists of the investing trophoblast in the form of a single layer of cells, attached to the inner surface of which, and occupying the upper hemisphere is the embryonal knot or inner cell-mass formed by the central cells of earlier stages.

The trophoblastic cells are now rather more flattened in form, especially where they bound the blastocoele. Four of them are in process of division. In one, in addition to the normal nucleus and in contact with it, there is present a quite small accessory nucleus.

In the blastocyst-cavity there is present what appears to be a mass of cytoplasm about the size of a central cell but non-nucleated ; and on one side, in contact with the trophoblast, is a vacuolated and much degenerate cell with a pale-staining remnant of the nucleus. Further evidence is thus afforded of the occurrence of degeneration amongst the central cells. The embryonal knot consists for the most part of large cuboidal cells separated here and there and from .the trophoblast by intercellular spaces. They appear similar in their cytological characters except that some few of them tend to have more definitely contoured cell-bodies and stain rather more deeply with eosin than the others. Two such cells are seen at the lower border of the inner cell-mass in fig. 29, and shortly above them is another definitely contoured cell with an eosinophil cell-body. We suggest that possibly these eosinophil cells are entodermal cells, and that the process of segregation or delamination which results eventually in the formation of a connected layer of entoderm has already commenced.

A somewhat remarkable feature, the significance of which is not clear, is the occurrence in seven of the central cells of a small accessory nucleus adjacent to the normal nucleus and exhibiting obvious signs of degeneration, such nuclei being small and in most cases deeply staining and pycnotic. A number of cells both trophoblastic and central are in division.

We append here a brief mention of the two uterine eggs from Cat 4.6.19, one of them, A (34), being in the morula stage, the other, B (85), an early blastocyst.

Morula 34. Diameter, Oil x 009 mm. Morula, 0-076 x 0-069 mm. Zona. 0-012 mm.

This morula in its stage of development is very similar to morula 31. What we take to be the commencing blastocyst- cavity is an irregular cleft-like space which reaches the trophoblast at one end and extends up obliquely between the central cells to terminate near the equator. Certain of the central cells stain rather more deeply and possess more definitely contoured cell-bodies than the others, and may possibly be the entodermal mother-cells. One such cell is in mitosis.

Blastocyst 35. Diameter, 0-11 x 0-10 mm. Blastocyst, 008 x 0076 mm. Zona, 0-012 mm.

This blastocyst appears to be slightly in advance of blastocyst 33, inasmuch as its trophoblastic cells are distinctly more flattened and the blastocyst-cavity is more extensive, but the central cells are much less compactly arranged than in 33, and we question whether the specimen is quite normal. We mention it here because it provides unmistakable evidence of the presence of two varieties of central cells, viz. (a) definitely contoured cells, ovoidal or spherical, with relatively small nuclei and voluminous light-staining cell-bodies, often vacuolated and frequently with one or two minute accessory nuclei, and (b) smaller cells of irregular form with relatively large nuclei and less voluminous, more darkly-staining cell-bodies. Variety (a) appears to be more numerous than (b), otherwise we should have been inclined to regard it as entodermal, but since neither corresponds with the presumed entodermal mother-cells in morula 34 and blastocyst 33, and since we are not satisfied that the specimen is quite normal, we do not venture to suggest which of the two is ectodermal and which entodermal in significance.

2. Formation of the Didermic Blastocyst

Blastocyst 36 (6 A. 6.5.12). Diameter in fixative, 0-25 mm. ; in section, 0-225 mm. Zona, 0-0036 to 0-0048 mm. Figs. 30 and 32, PI. 28.

This blastocyst is, unfortunately, separated by a considerable gap from blastocyst 33. In the interval marked growth has taken place, the diameter of the blastocyst having practically been trebled, whilst the constituent cells of the inner cell-mass have become segregated to form the embryonal ectoderm and the definitive entoderm. The wall of the blastocyst at this stage is trilaminar over the small area at the embryonal pole corresponding to the site of the inner cell-mass and unilaminar over the rest of its extent, where it is formed of the trophoblast alone. In all these later blastocysts, owing to the pressure exerted by the fluid filling the blastocyst-cavity, the blastocyst wall lies closely applied during life to the inner surface of the zona.

The trophoblast is now in the form of an attenuated layer of flattened cells which is somewhat thicker immediately around the embryonal primordium than elsewhere, whilst over the latter, where it constitutes the covering trophoblast (Eauber's layer), it is already extremely thin, though still perfectly continuous (figs. 30 and 32, PI. 28).

The embryonal primordium appears in section as a lenticular mass (0-096 x 0-099 mm. in diameter) situated at the upper pole, in close contact with the covering trophoblast. Its two constituent parts, the embryonal ectoderm and the entoderm, are now perfectly distinct. The embryonal ectoderm consists of a mass of cells, somewhat loosely arranged, with poorly defined outlines and relatively large nuclei. They contain numbers of small fat-globules. The entoderm is represented by a more or less connected layer of plump fusiform cells, closely investing the under-surface of the embryonal ectoderm and conterminous therewith. The cells possess definitely contoured bodies, and their cytoplasm and nuclei stain rather more deeply than those of the ectoderm, the nuclei being on the average smaller than those of the latter. They contain but few fat-globules. At its periphery the entoderm is in places attached to the trophoblast, but it has not yet begun to spread below the latter (figs. 30 and 32).

Over the margin of the embryonal ectodermal mass, the thicker trophoblast surrounding the latter thins out to form extremely attenuated covering trophoblast, the nuclei of which are small and flattened (fig. 80,

It is worthy of note that the zona is perceptibly thicker over the upper polar region (0-0048 nim.) than over the lower (0-0036 mm.), an indication of greater growth-activity in the latter as compared with the former.

Blastocyst 37 (6B. 6.5.12). Diameter, 0-33x0-32 mm. Diameter embryonal ectoderm, 0-072 x 0069 mm. Fig. 31, PI. 28.

This blastocyst differs in some slight details from the preceding and is a little more advanced. The trophoblast over the upper polar region is distinctly thicker than the corresponding area in 36, and is asymmetrical with reference to the embryonal primordium, the latter lying just within its margin.

The mass of embryonal ectoderm has now assumed a more rounded form, appearing in fig. 31 as a spherical mass of cells, whilst the nuclei show a tendency to become arranged round the periphery bordered by the entoderm. The cells contain numbers of fair-sized fat-globules and several of them are in mitosis.

The entoderm, composed where it underlies the ectodermal mass, of a single layer of fusiform to oblong cells, is now thickened round the periphery of the mass, so as to fill up the angle between it and the trophoblast. The cells are here cuboidal, in places two deep, and some of them are in division (fig. 31). 'Since this thickening directly underlies the trophoblast we may take it that we have here the first step in the peripheral growth of the entoderm to line the blastocyst-cavity. The entoderm contains sparse fat-globules, and such also occur in the thicker trophoblast.

The covering trophoblast is thick over the periphery of the ectodermal mass, but becomes markedly attenuated directly over the central part of the latter, its nuclei being small, ovoidal to flattened, and deeply staining. It appears to be continuous right across in the section figured (fig. 31), but in the next section there is a minute surface indentation which possibly marks an interruption in its continuity. In the next two sections continuity of the layer is also doubtful, but in the fourth succeeding that figured it is undoubted.

The zona over the lower hemisphere is just half the thickness (0-0024 mm.) of that over the upper.

Bias to cyst 3 8 (14.1.10 D).

Diameter, 0-35x0-33 mm. Diameter embryonal ectoderm, 0-072 x 006 x 004 mm. in thickness. Zona over embryonal primordium, 00036 mm., over lower hemisphere, 0-033 mm. Figs. 33 a, 33 b, 33 c, PI. 28.

This blastocyst, of slightly greater diameter than the preceding, is distinctly more advanced in development. The mass of embryonal ectoderm is still of a rounded form but is now more flattened on its upper surface, and on this there is present centrally a slight but definite depression, roofed over by the zona alone (fig. 33 a). The depression is limited to the section figured with a trace in the next following. Over it the covering layer has now definitely disappeared, and the peripheral trophoblast has become attached round its margin. It contains traces of a granular material, possibly the degenerate remains of the covering layer.

An important advance in the differentiation of the ectodermal mass is now seen in the assumption by its constituent cells of a fairly definite columnar arrangement, the cells being disposed radially to the curved floor of the surface depression, with their nuclei in two rows, a basal and a more central. Some of the cells are in mitosis. We think the surface depression is simply the outcome of this assumption by the ectodermal cells of a columnar arrangement. It has only a transitory existence and is not to be regarded as of the nature of a primitive amniotic cavity.

The entoderm invests the convex under-surface of the ectodermal mass as a continuous layer of more or less flattened fusiform cells, but an important advance is seen in the fact that it has now spread out for a short distance (0-072 mm.) below the trophoblast surrounding the embryonal primordium (figs. 83 a, 33 b, and 33 c).

Small fat-globules are sparsely present in the embryonal ectoderm, the entoderm and the trophoblast around the embryonal primordium. The latter portion of the trophoblast in this blastocyst is only slightly thicker than the remainder, and the zona is also more uniform in thickness than in 37.

Blastocyst 39 (18A). Diameter, 0-297 mm. Diameter embryonal ectoderm, 0-072x0-72 mm.

This blastocyst, the smallest of three from one cat, generally resembles the preceding. The mass of embryonal ectoderm is plano-convex in form, being more flattened and thinner than in 38, whilst the surface depression is shallower and more extensive. In both this and the preceding blastocyst, curiously enough, the depression is eccentric in position. The peripheral extension of the entoderm is just beginning.

Blastocyst 40 (18 C). Diameter, 0-34x0-32 mm. Diameter embryonal ectoderm, 0-048 x 0062 mm.

The embryonal ectodermal mass is exceptionally small and has the form of a concavo-convex disc, its surface depression being deeper and the columnar arrangement of its cells better marked than in 39. The entoderm has extended for a distance of 006 mm. beyond the margin of the embryonal ectoderm.

The zona and trophoblast, over an area of the upper hemisphere in which the embryonal primordium is again excentrically situated, are much thicker than over the lower hemisphere.

Blastocyst 41 (14.1.10 C). Diameter, 0-32 mm. Diameter embryonal ectoderm, 0-072 x 0-06 mm. x 0-03 mm. in thickness.

This blastocyst essentially resembles 88. The surface depression on the embryonal ectoderm is shallow and confined to two sections. The entoderm is just beginning to extend peripherally.

Blastocyst 42 (18B). Diameter, 0-32. mm. Diameter embryonal ectoderm, 0-072 x 0076 mm. x 0-03 mm. thick. Fig. 34, PI. 28.

This excellent blastocyst shows a distinct advance on the preceding stages in that the embryonal ectoderm now forms a definite slightly curved plate or embryonal shield, thicker centrally than marginally and composed of columnar cells (fig. 34). It is separated from the zona by a shallow but wide depression (0-048 x 0-004 mm. in diameter), at the periphery of which the trophoblast passes into continuity with the embryonal ectoderm. The entoderm consists of a continuous layer of flattened cells, which tend to be thicker below the margin of the ectodermal plate than centrally. It extends out below the trophoblast for a maximum distance of about 007 mm. The zona and" trophoblast are again thickest over a considerable area of the upper hemisphere, the embryonal primordium being excentric in relation to it. Fat-globules are present in fair abundance in the cells of the ectodermal plate and more scantily in the entoderm and thickened trophoblast.

Blastocyst 43 (11.2.10).

Diameter, 0-34 x 0-3 mm. Diameter embryonal ectoderm, 0096 x 0-09 mm. . x 0-026 mm. in thickness. Fig. 35, PI. 29.

This blastocyst essentially resembles the preceding but the embryonal shield has increased in area and become slightly thinner, whilst the entoderm, which is composed of a thin layer of flattened cells and appears to be uniform throughout, has now spread peripherally over about the upper third of the extent of the trophoblast.

The zona is, exceptionally, a shade thicker over the lower hemisphere than over the upper, whilst the trophoblast is pretty uniform over its extent.

Blastocyst 44 and 45 (29.4.12, 8 A and B).

3 A. Diameter, 0-52x0-47 mm. Shield-ectoderm, 009 x 0-099 mm.

3B. Diameter, 0-59x0-57 mm. Shield-ectoderm, 0-09 x 0-10 mm. x 0-024 mm. thick. Fig. 37, PI. 29, and Text- fig. 9.

These two blastocysts are very similar, 3 B being the better of the two. The most interesting feature in this stage is the presence of a locally thickened area of entoderm underlying the anterior portion of the embryonal shield, which we identify as the protochordal plate of Hubrecht (27) and which we shall speak of as the prochordal plate. As the result of the differentiation of this plate the embryonal area acquires, as 0. van der Stricht (54) has pointed out for the embryonal area of the Dog's blastocyst, a recognizable bilateral symmetry and also a recognizable antero-posterior axis (coincident with the line of symmetry), since the plate always lies nearer what is the anterior margin of the embryonal shield.

In the case of blastocyst 3 B our serial sections appear to be pretty accurately transverse to the antero-posterior axis. The shield-ectoderm forms a plate almost but not quite circular, its antero-posterior diameter being a trifle shorter than its transverse. The upper surface of the plate is practically flat, its under surface convex. It is composed of a single layer of columnar cells, thickest centrally and becoming thinner towards the margins where it becomes continuous with the trophoblast, the junction between the two being perfectly definite (fig. 37).

A graphic reconstruction of the prochordal plate in 3 B is shown in Text-fig. 9, from which it will be seen that it takes the form of a transversely oval area, situated excentrically below the central region of the embryonal shield and nearer the anterior than the posterior margin of the same. It begins 0-018 mm. behind the anterior margin of the shield and extends back for a distance of 0048 mm., attaining a maximum width of 0-07 mm. The entoderm forming the plate, has a thickness of 0007 mm. as compared with 0005 mm. for that underlying the hinder third of the shield. It is composed of small, almost, cubical cells with close-set ovalish nuclei (fig. 37) as described and figured (fig. 30, PI. xxxvii) by Hubrecht (27) for Sorex, and is thus readily distinguishable from the remaining entoderm in which the cells are larger and more flattened, their nuclei consequently being set farther apart. The plate lies below the thicker central part of the shield-ectoderm, and thins out on each side below the marginal region of the same.

Text-Fig. 9. Graphic reconstruction of shield-ectoderm and prochordal plate of blastocyst 45. x 500.

The entoderm has made further progress in its peripheral extension and now reaches the equator of the vesicle. The trophoblast over the upper hemisphere is still distinctly thicker than that over the lower, the former having a thickness of 0008 mm. the latter of 0-0048 mm.

Blastocysts 46 and 47 ( A and B).

4 A. Diameter, 0-52 mm. Shield-ectoderm, 0-108 x 0-108 mm. 4B. Diameter, 0-56 mm. Shield-ectoderm, 010 x 0-108 mm.

These blastocysts differ in no essential respect from the preceding, but the peripheral extension of the entoderm is not so great. A graphic reconstruction of the prochordal plate in 4 A is shown in Text-fig. 10.

It is essentially similar to that of the preceding blastocyst, appearing as a transversely oval area underlying the central region of the embryonal shield but more approximated to the anterior margin of the same. The peripheral embryonal entoderm around the plate is somewhat thicker than the extra- embryonal.

Blastocysts 48 and 49 (11.3.10 A and B).

Diameter, A, 0-5x0-48 mm. B, 0-6x0-5 mm. Shield-ectoderm, A, 0-11 x 0-12 mm. ; B, 0-12 x 013 mm.

The only feature in these blastocysts calling for comment is the position of the prochordal plate. It is now distinctly excentric in position and underlies the anterior two-thirds or thereabouts of the embryonal shield, much as in Text-fig. 11. Peripherally the entoderm has extended to the equator of the vesicle.

Blastocyst 50 ( C).

Diameter, 0-59x0-54 mm. Embryonal shield, antero-posterior diameter, 0-12 mm. ; transverse diameter, 0-10 mm. Fig. 18, PI. 26.

This vesicle is interesting in possessing a broad equatorial band of specially thickened trophoblast, suggestive at first sight of the precocious development of the zonary band of thickened placental trophoblast characteristic of much later stages but probably only indicative of the commencement of the general thickening of the trophoblast all over its extent which is already completed in the next older blastocysts.

The cells composing the band are cubical, with spherical nuclei, and vary in thickness from 0-007 to 0-009 mm. Over the lower pole the trophoblast cells are flattened and attenuated, whilst round the embryonal shield they are somewhat thicker, but even so are less than half as thick as those of the zonary band.

The shield-ectoderm and the underlying prochordal plate are seen in longitudinal section in fig. 18. The plate occupies much the same position as in the preceding blastocyst. It begins shortly behind the anterior margin of the shield-ectoderm and extends back for a distance of 0-08 mm. It is composed of cubical cells with close-set oval or spherical nuclei and attains a thickness of about 0-008 mm. The marginal embryonal entoderm around the plate has a thickness of about 0-005 mm. It continues out a short distance beyond the margin of the shield before passing into the thin extra-embryonal entoderm of the vesicle wall. The latter entoderm now extends a little below the equator. The zona over the upper hemisphere has a thickness of 0-0024 mm., and is slightly thicker than that of the lower hemisphere.

Blastocysts 51, 52, and 53 (11.3.15 A, B, C).

A. Diameter, 1-25x0-86 mm. Embryonal shield, 0-49 x 0-33 mm. x 0027 mm. in thickness. Zona about 00018 mm.

B. Diameter 1-4x1 mm. Embryonal shield, 0-41 x 0-3 mm.

C. Diameter 1 x 0-8 mm. Embryonal shield, 0-28 x 0-29 mm. x 0-027 mm. thick. Zona, 0002 mm. . Figs. 36 and 3S. PI. 29.

These three blastocysts, the latest of the Cat that we describe, are now bilaminar throughout, the entoderm having spread so as to form a complete lining to the blastocyst-cavity.

From the measurements given above it will be seen that in A and B the embryonal shield has increased in size, most markedly along its antero-posterior axis, and so from being approximately circular has assumed an oval form.

It will also be observed that in these two blastocysts one diameter is greater than the other so that they are no longer spherical but ovalish in form, and are thus on the way to assume the citron-shape characteristic of the later blastocysts of both the Cat and Dog. According to Bonnet (14), the long axis of the embryonal area in the Dog lies at right angles to the long axis of the vesicle, but we are unable to say what the relation is in these vesicles. In C, the smallest of the three, growth has also taken place but to a lesser extent and more uniformly, the blastocyst being still approximately spherical and the shield circular. A longitudinal section through the embryonal shield and prochordal plate of C is represented in fig. 38, PI. 29. The embryonal shield appears as a flat plate of fairly uniform thickness, intercalated in the trophoblast and composed of columnar cells, the nuclei of which in the central region are arranged in two irregular rows.

A feature of interest in these blastocysts is the general increase which has taken place in the thickness of the trophoblast. It now consists of a uniform layer, 0-009 mm. in thickness, composed of cubical to oblong cells with convexly projecting outer surface (fig. 36, PI. 29).

The prochordal plate (Text-fig. 11 and fig. 38) occupies the same position as in the immediately preceding blastocysts. It begins in front below the anterior margin of the shield- ectoderm and extends back below the anterior two-thirds of the same for a distance of 0-19 mm.

It has a thickness of 0012 mm. as compared with 0-007 mm - for the embryonal ectoderm around it, both being thicker than the corresponding parts in blastocyst 50. As in the latter, the embryonal entoderm continues out for a short distance beyond the margins of the shield before passing into the attenuated extra-embryonal entoderm of the vesicle wall (fig. 36).

In his paper on the structure of the blastocyst of the Cat, Schafer (45) described the presence of a thin structureless membrane on the upper surface of the embryonal entoderm which he termed the membrana limitans hypoblastica and regarded as of entodermal origin. We have observed indications of this same membrane in these blastocysts, and consider that it is simply the upper cell-membrane of the entoderm which has become artificially separated (compare also in this connexion Weysse (57), Assheton (5, p. 353), and Keibel (32)).

Text-Fig. 11. Graphic reconstruction of shield-ectoderm and prochordal plate of blastocyst 53. x 250.

We conclude our description of the bilaminar blastocyst with a brief account of an excellently preserved blastocyst of the Dog, for which we are greatly indebted to our friend Dr. J. A. Murray, Director of the Imperial Cancer Research Fund. It differs in certain interesting respects from the comparable stage in the Cat.

Blastocyst 54. Dog.

Diameter about 1-75 mm. Embryonal shield, 0-27 x 026 mm. Pigs. 89 and 40, PI. 29 ; fig. 19, PI. 26 ; and Text-fig. 12.

As in our oldest cat blastocysts, the blastocyst wall is bilaminar over its extent. The trophoblast, composed of large, flattened cells, is, however, extremely attenuated as compared with that of the Cat, whilst the extra-embryonal entoderm is also very thin (fig. 40).

The blastocyst of the Dog differs, as is well known, from that of the Cat in the presence externally to the zona of a gelatinous layer produced into villous processes (fig. 19). This layer was first described and figured by Bischoff (13) and later by Bonnet (14), who termed it the ' prochorion ' and showed that it served for the temporary attachment of the blastocyst. He regarded it as formed by a secretion of the uterine glands, but more recently 0. van der Stricht (53) has stated that it is derived from the superficial layer of the zona.

The shield-ectoderm is approximately circular and appears as a well-defined flat plate which is somewhat thinner marginally where it becomes continuous with the trophoblast than elsewhere. It is composed of columnar cells, the nuclei of which tend to be arranged in two irregular strata.; some of them are in process of division.

The most interesting structure in this blastocyst is undoubtedly the prochordal plate, a graphic reconstruction of which is shown in Text-fig. 12. As in our earlier cat blastocysts, it underlies the central region of the shield-ectoderm but nearer the anterior than the posterior margin of the same. It begins about 0-048 mm. behind the anterior margin of the shield and extends back for a distance of about 0-14 mm. with a width of about 0-10 mm. It differs markedly in its sectional appearance from that of our oldest cat blastocysts since it consists of a somewhat irregular layer of cells of variable form (branching, cubical, or fusiform) and with rounded or ovalish nuclei, smaller than those of the ectoderm. Its under surface presents an even contour but its upper surface is quite irregular, many of the cells being produced into branched and anastomosing processes which terminate on the under surface of the shield-ectoderm (figs. 39 and 19). A number of the cells are in mitosis, the axes of the spindles being at right angles or oblique to the plane of the entoderm (fig. 19), whilst here and there detached cells are met with lying free between the plate and the shield-ectoderm (figs. 89 and 19). These cells have evidently been proliferated from the plate, and we conclude accordingly that the proliferation of mesenchyme from the prochordal plate has already commenced at this relatively early stage.

Text-Fig. 12. Graphic reconstruction of shield-ectoderm and prochordal plate of blastocyst 54 (Dog). x 250.

Behind the prochordal plate the embryonal entoderm underlying the hinder portion of the shield is quite thin and inactive-looking (fig. 40, PI. 29).

Bonnet (14) apparently failed to recognize any localized thickening of the entoderm in the earliest blastocyst of the Dog he was able to examine (diameter 1-5 x 1-2 mm., embryonal shield 0-16 x 0-08 mm.), and even in blastocysts of his Group III (with diameters of 1-8-2-5 x 1-5-2 mm., the embryonal shield varying from 0-25x2 mm. to 0-4x0-35 mm.) he bases his orientation of the embryonal area on the fact that the long axis of the latter lies at right angles to the greater diameter of the blastocyst, and states that it is impossible to recognize with certainty the future anterior and posterior ends.

It is not until the blastocyst has attained a diameter of 4 - 5 x 3 mm. (with an embryonal area 0-65x0-75 mm. in diameter and a primitive streak 0-3 mm. in length) that he describes under the designation of the ' Erganzungsplatte des Urdarmstranges ' a very short somewhat thickened area, of the entoderm which underlies the anterior margin of the embryonal shield and is joined by the cranial end of the head process (' Urdarmstrang '). This area is actively proliferating mesoderm, and is recognized by Bonnet as identical with Hubrecht's protochordal plate.

0. van der Stricht (54), in a preliminary account of his observations on a series of early blastocysts of the Dog, states that in blastocysts 1-1-5 mm. in diameter the flattened entodermal cells ' sooner or later become columnar in the future cephalic half of the embryonal area. In this way the latter acquires its bilateral symmetry.' This thickened area undoubtedly corresponds with the area we have identified as the prochordal plate in the cat blastocyst. His statement (53, p. 494) that it has nothing to do with the formation of mesoderm is probably explained by the fact that he had not studied in detail blastocysts over 1-5 mm. in diameter.

E. van Beneden also clearly recognized the existence of this thickened area of entoderm in Vespertilio and at a remarkably early period. In his posthumous paper of 1911 (11), in the description of fig. 56 representing a section through an early blastocyst at the stage when the entoderm is already present in the upper hemisphere and the embryonal ectoderm is still in the form of a solid undifferentiated mass, special attention is called to the fact that below one-half of the ectodermal mass the lecithophoral layer is composed of very flattened cells, whilst below the other half the cells are almost cubical, and it is emphasized that this difference in thickness confers on the embryonal region of the blastocyst a bilateral symmetry. The same localized thickening is followed through older blastocysts up to the appearance of the primitive streak and head-process (fig. 68), and it is conclusively demonstrated that the thickening underlies the cranial part of the shield-ectoderm, that the primitive streak differentiates from the more caudal region of the same, underneath which the lecithophoral layer is thin, and that the head-process extends forwards, at first as a free process, as far as the hinder margin of the thickening. Later on, definite continuity is established between the two.

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1924 Cat Development: 1. Ovum of the Cat | 2. Process of Cleavage | 3. Formation of the Blastocyst | 4. Discussion | Plates | cat


Hill, J. P., and Tribe, M. 1924. The early development of the cat (Felis domestica). Quart. J. Microsc. Sci, 68, 513-602.

Cite this page: Hill, M.A. (2024, April 24) Embryology The early development of the cat 3. Retrieved from

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