The early development of the cat 2
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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 II.—The Process of Cleavage
1. Description of Cleavage
Our material of the early cleavage stages is scanty, but such as it is, it supplements that of R. van der Stricht. The latter was not able to observe the first cleavage mitotic figure. We have been a little more fortunate, having obtained from one cat three eggs in the phase of the first cleavage, but unfortunately in two of them the spindle is not well preserved and it is impossible to determine its axis. The third egg (egg 12, B. 16.5.19) is worthy of a brief description.
Diameter, 0-093x0-084 mm. Ovum, 0-079x 0074 mm. Zona about 0003 mm. Fig. 3, PI. 24.
The ovum is incompletely divided into two halves by a deep circular groove extending in more than half-way towards the centre (fig. 3). In each half there is a small membranate nucleus, the nuclei differing in character. That on the right (0-008 x 0-006 mm. in diameter) is oval and possesses one karyosome and peripheral chromatin granules; that on the left is slightly larger (0-009x0007 mm.) and more deeply staining than the other, it possesses several karyosomes, and its contour is somewhat irregular. In the centre, about mid-way between the two nuclei, remains of the spindle-fibres with their intermediary thickenings (rsp.) are distinctly visible. Polarity in the disposition of the fat-globules is not obvious, but it may be noted that the right half appears to be richer in fat than the left.
The two polar bodies lie in the plane of division, one, the larger (measuring 0-007 x 0-0048 mm.), lies peripherally in contact with the left half; the other, smaller, lies opposite the groove.
Two-celled Egg in Division.
Egg 13 (A. 15.5.19 (a)).
Diameter, 0-009 mm. Fig. 4, PI. 24. This egg is one of three and is belated as compared with its companions which are in the four-celled stage.
The two blastomeres are of unequal size ; the larger one (on the left in fig. 4) has a diameter of 0-07 x 0-04 mm. and is richer in fat-globules than the other which measures 0-06x 0-03 mm. The larger blastomere possesses an equatorial plate of clumped chromosomes, practically centrally situated and with remains of the asters on opposite sides, but the spindle- fibres are not preserved. The mitotic figure has been cut obliquely, and it is not possible to determine with certainty the plane of division, though vertical division approximately at right angles to the plane of the first cleavage is suggested. The smaller blastomere possesses a membranate nucleus of a distinctive lobed appearance situated subcentrally towards the plastic pole. Its nuclear membrane is very delicate and irregularly sinuous or lobed. The nuclear space is occupied by a dense spireme or group of chromosomes except peripherally, below the nuclear membrane, where the reticulum appears to have become swollen and vesicular. Adjacent to the nucleus is what seems to be an extra-nuclear group of chromatin granules as well as a small accessory nuclear body, whilst a second similar body is present towards the plastic pole. We agree with E. van der Stricht that these nuclear bodies are to be regarded as lobes of the nucleus which for some unknown reason have become separated off. In this same blastomere there is present at the deutoplasmic pole a minute spherical body with a deeply staining membrane enclosing a lighter area in which are situated two central granules. This is probably the ' corps enigmatique ' which E. van der Stricht thinks is characteristic of this particular blastomere with the lobed nucleus, but we find a similar body with one central granule situated at the plastic pole of the other blastomere.
As regards the polarity of the blastomeres, in both the fat-globules are more abundant towards one pole, the deutoplasmic pole which corresponds in the two. They extend up peripherally towards the opposite or plastic pole, but leave that, as well as the central region, relatively free.
The superficial groove between the two blastomeres, coincident with the plane of cleavage, is occupied by a granular material which may possibly have been eliminated by the blastomeres, and, if so, is to be regarded as deutoplasmolytic in nature.
It is clear from the condition of the nuclei in this egg that the larger blastomere in Avhich the equatorial plate is already differentiated would have divided before the other ; this is in agreement with E. van der Stricht's conclusion that the blastomere with the regular (non-lobed) nucleus divides before that with the lobed nucleus.
Longley (37) figures a two-celled egg (his fig. 13, PI. iv) in which the nuclei of the two blastomeres are seen to differ in size, one is larger and spherical in outline, the other is smaller and irregularly ovalish. A polar body is shown lying in the peripheral groove between the blastomeres. The two-celled egg of the Dog, according to 0. van der Stricht (52) is very similar, apart from the form and disposition of the mitochondria, to that of the Cat. The blastomeres exhibit a distinct polarity in the distribution of the fat-globules, whilst their nuclei, excentrically situated towards their upper poles, differ in form, one being distinctly lobed, the other, more regular. The two polar bodies lie adjacent to the lower poles of the blastomeres.
E. van der Stricht lays emphasis on the facts that the two blastomeres of the two-celled egg differ in the form of their nuclei, in the time of their division, sometimes in the number of the larger rnitochondrial formations occurring in them, and always in the presence of the ' corps enigmatique ' in the blastomere with the lobed nucleus (but in regard to the latter compare above). He apparently attaches considerable importance to these differences, and points out that they afford confirmation of the conclusion reached by E. van Beneden in the case of the rabbit that the first two blastomeres play perfectly distinct roles in the formation of the constituent parts of the blastocyst. We need only say here that that conclusion is not in accordance with our observations which lead us to think that the parent cell of the inner cell-mass is not separated until the next cleavage, i. e. is one particular cell of the second cleavage generation, the other three cells furnishing the trophoblast.
Two three-celled eggs have been described by E. van der Stricht (55, p. 462). The blastomere of the two-celled stage with the regular nucleus has divided, whilst the other with the lobed nucleus and the ' corps enigrnatique ' is still undivided, its nucleus being at the beginning of the prophase and containing a spireme-thread.
It is clear that the two divisions of the second cleavage are not synchronous but successive. If the first cleavage plane, which passes through the plastic and deutoplasmic poles of the ovum be regarded as vertical, then according to E. van der Stricht's observations the first of these two divisions of the second cleavage is also vertical and at right angles to the first cleavage plane. Assuming this determination to be correct, it follows, from our own observations on the four-celled stage, that the plane of the second of these divisions must be horizontal (equatorial), since in the four-celled stage the blastomeres are in pairs so arranged as to form a cross-shaped group, the plane of division of the one pair being at right angles to that of the other pair. That being so, it necessarily follows frorn the polar distribution of the fat-globules in the blastomeres of the two-celled stage as described by E. van der Stricht and by us, that one of the four blastomeres of the four-celled stage should be poorer in fat-globules and one richer in the same than the other three.
Four-celled Eggs. E. van der Stricht had available two four-celled eggs, but the arrangement of the blastomeres is not described. We have examined four such eggs, two from one cat, egg 14 (A. 15.5.19 b) and egg 15 (A. 15.5.19 c), and two from another, egg 16 (3.6.19 a) and egg 17 (3.6.19.beta).
Egg 14. Diameter about 009 mm.
Two views of the model of this egg are shown in Text-fig. 1 a and b. The four blastomeres are readily grouped into two pairs (1 & 2) and (3 & 4), so arranged as to form an interlocking cross-shaped group.
Text-Fig. 1. Two views (a and &) of model of egg 14, four-celled stage. x about 300.
Pair 1 & 2 occupied one hemisphere and pair 3 & 4 the other, the plane of division between 1 & 2 cutting that between 3 & 4 obliquely. The blastomeres are approximately of the same size, but one pair is richer in fat-globules than the other. Apart from this no one blastomere can be said to be specially poor or specially rich in fat-globules.
Egg 15. Text-fig. 2 shows two views (a and b) of the mode, of this egg, which is noteworthy on account of the inequality in the size of the blastomeres.
Text-Fig. 2. Two views (a and b) of model of egg 16, four-celled stage. x about 300.
Blastomeres 1 & 2 and 3 & 4 form pairs, the plane of division between 1 & 2 being at right angles to that between 3 & 4, so that there is an obvious cross-shaped arrangement, less regular than in egg 14. Blastomeres 3 & 4 together are considerably larger than those of the other pair, pointing to inequality in the two-celled stage, and the facts that blastomere 4 is the largest and 2 the smallest point to inequality in both divisions of the second cleavage. Blastomere 4 is fairly rich in large fat-globules, whilst 3 has very few, from which we may conclude that the plane of division between these two blastomeres is that of the horizontal second cleavage.
Egg 16. Diameter, 0-08 x 0-072 x 0-072 mm.
Blastomeres 1 & 2 and 3 & 4 form pairs so arranged as to form a perfectly definite cross (Text-fig. 3, a and b), the plane of separation of 3 & 4 being at right angles to that of 1 & 2. Blastomeres 1 & 2 extend round so as to envelop 3 & 4 laterally. What appears to be a polar body lies in the plane of division between 1 & 2 and may mark the vertical axis. If so, then the division between 3 & 4 again marks the horizontal second cleavage plane. The blastomeres apart from slight differences in size appear to be similar, though possibly blastomere 3 is somewhat richer in fat-globules than 4.
Text-Fig. 3. Two views of model of egg 16, four-celled stage, x about 300.
Egg 17. Diameter, 0-088 x 074 x 0-054 mm.
This egg is from the same cat as the preceding egg but is smaller.
Blastomeres 2 & 4, which are large, and 1 & 3, which are smaller and differ in size (1 being larger than 3), seem to form pairs of very unequal size. They are arranged to form a rather irregular cross, the plane of division between 1 & 3 being at right angles to that of 2 & 4. A minute oval body in blastomere 3 is probably the ' corps enigmatique '. In all these four-celled eggs there is present in the superficial grooves between the blastomeres, and between the latter and the zona, a granular material which is possibly deutoplasmolytic.
We have available for description two seven-celled eggs.
Egg 18 (13.5.19) and egg 19 (184.108.40.206). E. van der Stricht records obtaining four eggs (from the Fallopian tube 0-5 cm. from its uterine opening) with seven or eight blastomeres, and he figures sections of two of those with seven blastomeres (his photo. 87, 88, 89). This seven-celled condition results from the fact that three of the blastomeres of the four-celled egg have undergone the third cleavage divisions, whilst one has remained undivided. The fact that this stage is represented in the collections of both E. van der Stricht and ourselves may be taken to indicate that the belated division of one of the blastomeres of the four-celled stage is a normal occurrence, and accordingly the question arises as to its significance. We suggest that possibly this particular blastomere may turn out to be the mother-cell of the two central cells of later cleavage stages, from which the inner cell-mass or embryonal knot originates.
Egg 18 (13.5.19). Diameter inside zona, 0-082 x 0069 x 009 mm.
An excellently preserved egg but exceptional in that the zona is separated into two layers, the intervening space containing coagulum in which are sperms and sperm-heads. The seven blastomeres are arranged as shown in the figure of the model (Text-fig. 4 a and b). There are six smaller blastomeres (1-6) and one larger (7), measuring 0036 x 0-055x0-05 mm. This, an undivided blastomere of the four-celled stage, is situated at one pole of the oval egg, and is overlapped by blastomeres 6, 3, and 5 and to a very slight extent by 4. Blastomere 3 is in the telophase of division, the two groups of chromosomes being separated by the thickness of one section (figs.. 10 and 11, PI. 26), and is superficially grooved round its mid-region (Text-fig. 4fc), i.e. this blastomere is in process of completing the fourth cleavage division.
Text-Fig. 4. Two views (o and 6) of model of egg 18, seven-celled stage. Blastomere 7 is regarded as the parent central cell, x about 300.
The blastomeres all contain numerous larger and smaller fat-vacuoles and, in the superficial grooves between the blastomeres as well as in places between the blastomeres and the zona, there is present a granular material in which are small spherical masses, the whole probably deutoplasmolytic. A polar body (00096 x 0-007 in diameter) is present in the superficial groove between blastomeres 1 & 2, and rather deeply situated in blastomere 6, is a possible ' corps enigmatique '.
There has evidently been a certain amount of shifting of the blastomeres as is indicated by the overlapping of blastomere 7 by the other cells, and accordingly it is very difficult to make out the grouping of the blastomeres in pairs. The following grouping is suggested as a possibility, the Eoman numerals indicating the blastomeres of the four-celled stage :
|7 +||3 & 5 +||1 & 6 +||2 & 4|
The plane of division between 7 and 3 & 5 is approximately at right angles to that between 1 & 6 and 2 & 4.
Egg 19 (220.127.116.11). Diameter, 0-1 xO-09 xO-08 mm., distinctly larger than the preceding.
Two views of the model of this egg are shown in Text-fig. 5 a and b and a sectional view in fig. 20, PI. 27. The undivided blastomere (5) (Text-fig. 5 b) is large, measuring 0-072 x 0-069 x 0-045 mm. in diameter. Blastomeres 1, 2, 6, 7, and 4 lie in contact with its margin, but there is no definite overlapping. Blastomere 3 lies on the opposite side of the egg to 5 (Text- fig. 5 a), and is separated from it centrally by a small space.
Text-Fig. 5. Two views (a and 6) of model of egg 19, seven-celled stage. Blastomere 5 is regarded as the parent central cell, x about 300.
Blastomeres 6 & 7 are larger than either of the other pairs, G exceeding 7 in size. Blastomeres 3 & 4 are approximately of the same size as 1 & 2. Here again it is difficult to determine the grouping, but we suggest the following :
|5 +||6 & 7 +||1 & 2 +||3 & 4|
Under this grouping the plane of division between 5 and 6 & 7 is very oblique to that between 1 & 2 and 3 & 4. In the central space, as well as in the superficial grooves between the blastomeres, there is present a considerable amount of finely granular material.
The relations of the paired blastomeres to the undivided blastomeres in these two eggs, and more especially in 18, distinctly suggest the possibility of the occurrence in succeeding stages of a process of epiboly whereby the smaller blastomeres come to surround the larger one. In this connexion it is worthy of note that in the Pig, Assheton (4) states that one of the blastomeres in a nine-celled stage ' is far larger than any of the others ' (p. 335, and fig. 14, PL 26). He further states (p. 337): ' In the four-celled stage there is always a slight difference in size, but at the eight- to twelve-segment stage the difference is most marked, and recalls the great difference which we find in many molluscan ova. In no case, however, have I found any difference perceptible except in size. The smaller segments form a cap upon the larger.' He thinks it' extremely probable ' that a process of epiboly takes place, though in consonance with his well-known views he holds that ' it is really the hypoblast which grows round the epiblast ' (p. 338).
Three eggs of this stage are available for description, all obtained from one cat (18.104.22.168). They are of very great interest inasmuch as the two constituent parts of the future blastocyst are now clearly distinguishable. The future inner cell-mass or embryonal knot is represented by two centrally placed cells, the future trophoblast by fourteen peripheral blastomeres which enclose the central cells either partially or completely.
Egg 20 (8 C). Diameter inside zona, 009 x 0-069 x 0-07S mm. Zona thin, 00024 mm.
Four views of the model of this egg are shown in Text-fig. 6 a, b, c, d, and a combined view of two sections in fig. 21, PL 27. The egg, now in the stage of a morula, consists of fourteen peripheral or trophoblastic cells and two centrally placed cells which are all but completely enclosed by the former. The peripheral blastomeres are spherical and are in contact only over small contiguous areas of their surfaces. They contain fat-globules in fair abundance (fig. 21, PI. 27) and are similar though they differ somewhat in size, e. g. blastomere 2 measures 0028 x 0-024 x 0-018 mm. in diameter, blastomere 13, 0-028 x 0026x0026 mm.
Text-fig. 6. Four views, a, b, c, d (section), of model of egg 20, sixteen-eelled stage. Blastomeres 5 & 12 are the central cells, x about 300.
The two central blastomeres (Text-fig. 6 d, 12 & 5), in spite of their very different destiny, do not appear to differ from the peripheral in any recognizable respect except position. They are here nearly completely enclosed, but it is an interesting and significant fact that the outer end of central blastomere 12 is visible in the model (Text-fig. 6 c) through the gap at one pole of the mornla, bounded by blastomeres 13, 15, 14, and 16. Text-fig. 6 d, representing the cut surface of a vertical section of the model, illustrates this relation as well as the position of the central blastomeres (5 & 12), and affords conclusive evidence in favour of the occurrence of epiboly, i. e. of a process of overgrowth whereby the central cells become secondarily enclosed by the peripheral.
The two central cells differ slightly in size, blastomere 12 measuring 0-024x0-031 xO-24 mm., and 5, 0-018 x 0-028 x 0-024 mm. They are thus of very much the same average size as the peripheral cells. As concerns their origin we think their relations to each other and to the peripheral cells indicate pretty definitely that they have arisen by the slightly unequal division of a single parent cell which we suggest is the large blastomere of the seven-celled stage, representing an undivided blastomere of the four-celled stage.
If our suggestion is accepted then the constitution of the morula might be represented as follows :
(fix this table)
I II III IV
3rd 4th 4th 5th 4th 5th cleavage generation
From this lineage it follows (1) that the central cells ought to be larger than the peripheral, and (2) that four of the peripheral cells ought to be smaller than the remaining ten since they belong to a later cleavage generation, viz. the 5th. So far as we have observed neither of these conditions appears to be realized in this egg as the result perhaps of inequalities in the antecedent divisions.
There is of course another alternative, and that is to assume that normally an eight-celled stage occurs in the Cat and that the products of division of one of the blastomeres simply become overgrown and enclosed by the fourteen blastomeres arising from the division of the other seven. In this case the constitution of the morula would be as follows :
(fix this table)
I II III IV
1x2 + 1x2 + 1x2 + 1x2=8 blastomeres
(lx2C) + (l x2) + 4 + 4 + 4 = 2 0 + 14
i.e. the sixteen blastomeres would all belong to the fourth cleavage generation, and the parent cell of the inner cell-mass would belong to the third cleavage generation instead of to the second as in the first interpretation. Unfortunately no detailed description of an eight-celled egg is available, and a more precise determination of the lineage of the single formative cell must await the result of further observation.
Egg 21 (8 A). Diameter (inside zona), 0-091 x 0084 x 0-09 mm.
Text-fig. 7. Four views, a, b, c, d (section), of model of egg 21, sixteen-celled stage. Blastomeres 1 and 16 are the central cells, x about 300.
In this morula (Text-fig. 7 a, b, c, d) the peripheral cells form a rather more continuous enveloping layer than in the preceding. There is one undoubted central cell (Text-fig. Id, 16) completely, enclosed and measuring 0-03 x 0-026 x 0-024 mm. in diameter. It is thus of approximately the same size as central blastomere 12 of the preceding egg, and is too small to give origin by division to two centrals of the size of those in that egg. One of its ends, however, is in contact with a large blastomere (1) measuring 0-03 x 0036 x 0-084 mm., which projects at one pole of the morula, being surrounded by five of the peripheral blastomeres (Text-fig. 7 a and c (2, 3, 4, 5, 6). Enclosed between these blastomeres, it extends inwards to terminate in contact with central cell (16) (Text-fig. 7 d). We regard this blastomere (1) as likewise a potential central cell (the sister-cell of 16) which has not yet become completely enclosed by the overgrowth of the surrounding peripheral cells.
Egg 22 (8 B). Diameter (inside zona), 0096 x 0-079 x 0-078 mm.
This morula is very similar to the preceding. There is again one undoubted central cell measuring 0-033 x 0-028 xO-03 mm. in diameter. This central cell comes into contact with a blastomere (16) which projects at the surface and whose base is surrounded by five peripheral cells, as in the case of blastomere 1 of the preceding egg. Curiously enough, its measurements are identical with those of its presumed sister-cell, the undoubted central. Here again we think that the epibolic process is less advanced than in egg 20.
In these sixteen-celled eggs a good deal of granular coagulum is present around and between the blastomeres.
Twenty-two- to Thirty-one-celled Eggs
The four morulae included here are all characterized by the presence of two central cells, completely enclosed by a single layer of trophoblastic cells to whose division the increase in the number of the blastomeres is due. The epibolic process has accordingly been completed.
Egg 23 (22.5.19 A). Diameter, 0-10x0-084 mm. Morula, 0-09 x 0-07 x 0-08 mm. Zona, 0-007 mm., now markedly thickened. This morula consists of twenty-two cells, two central cells completely surrounded by twenty trophoblastic. The cells are compactly arranged, without intercellular spaces between them, and entirely fill the space inside the very thick zona (figs. 12 and 13, PI. 26).
On the assumption that the two centrals are derived from a single blastomere of the four-celled stage, the constitution of this morula may be expressed as follows :
I II III IV
(1 x 2 0) + (1 x 2 x 2 x 2) + (1 x 2 x 2 x 2) + (1 x 2 x
i.e. the morula consists of two cells of cleavage generation 3 (the two centrals), sixteen of cleavage generation 5, and four of cleavage generation 4. It follows that four trophoblastic cells should be larger than the remainder, and from our measurements we actually find that three at least are distinctly larger than the others.
The trophoblastic cells vary in diameter from 003 x 0-043 x 0-031 mm. (blastomere 14) to 0-018 x 0019 x 0-016 mm. (blastomere 13). They are rich in fat-globules, mostly of large size. Blastomeres 2 & 5 contain each a small accessory nucleus, and in 5 in addition there is present the ' corps enigmatique ' deeply placed to the inner side of the nucleus (fig. 12, 5. c.e.). Blastomeres 10 & 11 are sister-cells, recently formed as indicated by their small deeply staining nuclei, whilst blastomeres 16 & 18 are in process of division. Blastomeres 5 & 13 (figs. 12 & 13), though markedly unequal in size, follow on in the series in apparent continuity and probably also form a pair.
The two central cells lie in contact and are of unequal size, central cell 6 (fig. 12) measuring 0-02S x 0-026 x 0-024 mm. in diameter, central cell 12 (fig. 13) measuring 0024 x 0019 x 0-018 mm. They are less rich in fat-globules than the trophoblast cells. The relations of blastomeres 12 & 13, as seen in fig. 13, rather suggest that these blastomeres might be sister- cells, the products of tangential division but, as stated above, we think it more probable that blastomeres 5 & 13 are sister-cells.
Egg 24 (22.15.19 B). Diameter, 0-098x0-091x0-078 mm. Morula, 0-084 x 0-076 x 0-07 mm. Zona, 0-006 mm.
This very fine morula, from the same cat as the preceding, consists of twenty-three blastomeres, two centrals (11 & 13), and twenty-one trophoblastic. Three views of the model of this egg are shown in Text-fig. 8 a, b, and c, whilst three consecutive sections are figured in figs. 14-16, PL 26. As compared with the preceding egg the trophoblast cells here form a more regular enclosing layer around the centrals, their characteristic radial arrangement around the latter being well shown in Text-fig. 8 c and fig. 14.
Text-fig. 8. Three views, a, b, c (section) of model of egg 24, twenty-three-celled stage. Blastonieres 11 and 13 are the central cells, x about 300.
Blastomeres 1 & 2 are sister-cells, the division having but recently been completed as is evidenced by the small size of their nuclei and the still recognizable remains of the spindle. In blastomere 17 is a ' corps enigmatique ' superficially placed, whilst adjacent to the large nucleus is a small accessory one.
The two central cells again differ in size (Text-fig. 8 c and figs. 14—16, c.c 11 & 13). Central cell 11, the larger of the two, measures 0036 x 0033 x 003 mm. and possesses a membranate nucleus containing a segmented spireme, indicative of early division. Central cell 13 is much smaller, measuring 0-022 x 0-021x0-018 mm. Its nucleus is in the resting condition. In the cytoplasm of both are numbers of large fat globules.
Egg 25 (22.214.171.124). Diameter (in fixative), 0-1 x 0-09 mm. Zona thin, partially separated.
The morula consists of twenty-four cells, two central, and twenty-two trophoblastic (fig. 27, PI. 27). One of the centrals is slightly larger than the other and possesses two nuclei, a larger and a smaller.
Egg 26 (126.96.36.199). Diameter (in fixative), 0-127x 0-1 mm.
This morula consists of thirty-one cells, two central and twenty-nine trophoblastic. The trophoblastic cells do not form such a regular layer as in the preceding morula, the cells being more spherical in form.
The two central cells are in contact and are similar except that one is slightly larger than the other. E. van der Stricht (55) gives two microphotographs of sections of a thirty-celled egg (figs. 92 and 93, PI. xix) in which already several central cells appear to be present. It is noteworthy that in all of these four morulae (eggs 23-6) one of the central cells is larger than the other. It is tempting to suggest that the one (the larger since it appears to divide first (egg 24)) gives origin to the embryonal ectoderm, the other to the entoderm, but we have no evidence to offer in support of this suggestion.
A polar body with three chromatin masses (fig. 15, Pb.) lies in contact with the zona in the superficial groove between blastomeres 8 & 12.
Fifty-nine- to Sixty-three-celled Eggs
Egg 27 (27.1.10 B). Diameter, 040 x 0-09 mm. Morula, 0-076 x 0-072 x 0-08 mm. Zona very thick, 0-012 mm.
Egg 28 (27.1.10 A). Diameter, 0-10 x 0-098 mm.-. Zona also thick, 0-0096 mm. Fig. 26, PI. 27, and fig. 17, PI. 26.
These two morulae, from the same cat, are essentially similar, differing only in the number of the blastomeres. It has been a very difficult task to make an accurate count of the blatomeres, but we estimate that egg 27 consists of ± 59 cells of which 10 are centrals and 49 trophoblastic, whilst egg 28 consists of ± 63 cells of which S are centrals and 55 trophoblastic. The blastomeres are not so compactly grouped as in the preceding morulae, the central cells, in particular, being loosely arranged with obvious interspaces between them. The trophoblast consists of a single connected layer of ovoidal cells, clearly marked off from the centrals (figs. 26 and 17). They contain fat-globules and vary somewhat in size, their average diameter in egg 28 being 0-015 x 0-014 mm. One of them in the latter egg is in process of division. The trophoblast in both these eggs has over considerable parts of its extent shrunk away from contact with the zona.
The central cells are spherical or cuboidal in form, and on the average are smaller than the trophoblastic cells ; in egg 27 they vary in diameter from 0-012 to 0-018 mm., but we have not observed any staining or other difference between the larger and the smaller cells. They contain fat-globules and do not appear to differ in any essential respect from the trophoblastic cells. In two of them in "egg 28 there is present a small spherical body with a light centre and a darker-staining periphery, suggestive of the ' corps enigmatique '. In egg 27 one of the centrals is in process of division.
Unfortunately our records do not state whether eggs 27 and 28 were derived from the tube or the uterus ; if from the latter we think the fact would have been noted. R. van der Stricht (55) states that he obtained morulae of about twenty- eight to thirty blastomeres from the uterine third of the tube.
2. Remarks on Cleavage
Our observations on the cleavage of the fertilized ovum of the Cat may be summarized as follows.
The first cleavage which we assume to be vertical divides the ovum into two blastomeres which may be equal or somewhat unequal. One of the two possesses a curiously lobed nucleus, the other a normal one.
The second cleavage is effected by two successive divisions, the blastomere with the normal nucleus dividing before the other and in a vertical plane with resulting production of a three-celled stage, the other dividing later in the horizontal (equatorial) plane, at approximately right angles to the first. As the result the blastomeres of the four-celled stage are grouped in two pairs so arranged as to form a cross-shaped figure. This is apparently the usual arrangement of the blastomeres at this stage in the Monodelphia, having been recorded for a considerable number of forms, Dog and Eabbit (Bischoff), Rhinolophus (van Beneden and Julin), Vespertilio (van Beneden, 11, fig. 6, but not for the eggs shown in figs. 4 and 5), Erinaceus (Kunsemiiller), Macacus nemestrinus (Selenka). Assheton (6) states that he has seen four-segments stages of rabbit, pig, dog, ferret, and hedgehog, and has always found the pairs ' crossways '.
The third cleavage is represented in our material by two seven-celled eggs, three of the blastomeres of the four-celled stage having divided, the fourth remaining undivided. In one of these eggs (18), the large blastomere is already to some slight extent overlapped by the smaller.
Our next cleavage stage comprises three eggs composed of sixteen blastomeres. In all these we are able to recognize a grouping into two more centrally situated blastomeres and fourteen peripheral blastomeres which more or less completely surround the former. The peripheral cells we interpret as the future trophoblast, and the two central cells as the parent cells of the inner cell-mass. The central cells, we suggest, are derived by the division of the large blastomere of the seven-celled stage, i.e. from one of the blastomeres of the four-celled stage or alternatively by the division of one of the blastomeres of the eight-celled stage ; unfortunately no description of an eight-celled egg of Pelis is available. On the basis of the former suggestion the sixteen-celled stage would comprise two central blastomeres belonging to the third cleavage generation, ten blastomeres of the fourth generation, and four of the fifth generation. On the alternative suggestion, the sixteen blastomeres would all be of the same cleavage generation, viz. the fourth.
We conclude that the parent formative cell from which the central "cells and eventually the inner cell-mass are derived, is one particular blastomere either of the four-celled or eight-celled stage, i. e. it belongs either to the second or to the third cleavage generation.
Such a ' precocious segregation ' of the blastomeres into two groups, with determinate destinies, has now been recorded for a number of mammals, for the marsupial, Dasyurus (Hill, 24), where it is effected at the fourth cleavage, for Didelphys (Hartman, 23, Hill, 25), which is the only case definitely known to us amongst the mammals in which the segregation is effected at the first cleavage, and by various observers for Monodelphia. Hubrecht (28) has shown that the early morula of Tupaia, composed of about twelve or more cells, consists of a single lightly staining central cell completely enclosed by a single layer of peripheral cells. In the Sheep, Assheton (5) records that in two out of four eight-celled stages, ' one of the segments differs from all the others, from which he concludes that the differentiation occurs at the third generation, whilst in the Pig, as already noted (p. 542), he states that one of the blastomeres in a nine-celled stage ' is far larger than any of the others '. In the case of Erinaceus, Baurneister (7) describes the morula of about sixteen cells as consisting of a peripheral layer enclosing one or several central cells, whilst in Xantharpya, Kohlbrugge (33) figures, but oddly enough does not describe, sections of early morulae in which two categories of cells are evident. In his figs. 5 and 6 a single central cell appears, surrounded in the one case by five and in the other by seven peripheral cells ; in fig. 7 two centrals are visible, surrounded by ten peripheral; and in fig. 8, five centrals surrounded also by ten peripheral.
This early separation of the blastomeres into two groups respectively formative or embryonal and non-formative or trophoblastic is a phenomenon which Ave believe with van Beneden will be found to be characteristic of all the Monodelphia without exception, though the precise time of separation of the two groups probably varies somewhat in different forms. And that a process of overgrowth or epiboly actually does occur whereby the formative cell or cells become enclosed by the trophoblastic is, we think, conclusively demonstrated for the Cat by the text-figures of the models of early morulae that we present in this paper. The occurrence of such a process was originally described by E . van Beneden (8) for the Babbit in 1875, and subsequently by van Beneden (10) and Duval (20) for Vespertilio and by Assheton (5) for the Sheep, and it is doubtless also of universal occurrence in the Monodelphia though evidently not always easy to demonstrate, judging from the accounts of E . van Beneden himself and of Assheton.
Succeeding these sixteen-celled eggs, we describe four morulae composed of twenty-two, twenty-three, twenty-four, and thirty-one blastomeres respectively, in all of which the epibolic process has been completed, there being present two central cells of unequal size, completely enclosed by a single layer of trophoblastic cells to whose division the increase in the number of the blastomeres is due.
Active division of the trophoblast cells proceeds, and at the same time the central cells also begin to divide. R. van der Stricht figures sections of a thirty-celled morula in which already several central cells are present, and we describe two morulae with about fifty-nine and sixty-three blastomeres respectively, the former possessing ten central cells and the latter eight. Continued division of both the central and trophoblastic cells finally leads to the stage of the completed morula represented by our morula 29, in which the space inside the trophoblast is filled by a central group of cells representing the future inner cell-mass or embryonal knot and formed by the division of the two central cells of earlier stages.
3. Yolk-Elimination (Deutoplasmolysis)
This phenomenon, the elimination from the fertilized ovum or from t h e blastomeres during early cleavage stages of surplus deutoplasmic material, was first described by 0. van der Stricht (50) for Vesperugo under the name of ' deutoplasmolyse ' and independently by one of us (24) for Dasyurus. It has now been recorded for a number of other mammals.. Mouse (Lams and Doorme, 35), Cavia (Lams, 36), Dog (0. van der Stricht), Didelphys (Hartman, Hill), Cat (E. van der Stricht). The latter observer states that most of the eggs he examined in stages of cleavage (two-, three-, and many-celled) exhibited very evident signs of deutoplasmolysis, the fragments of deutoplasm, often in process of liquefaction, lying generally in the neighbourhood of the polar body. In several of the cleavage stages we have studied, we have recorded the presence of a granular material, occasionally as in egg 18 containing small spherical masses, around the blastomeres. This material, or some of it at least, is very possibly deutoplasmolytic, but we have not observed it in actual process of elimination, so that it is difficult to judge how much of it represents surplus material, how much coagulum, the result of fixation.
The elimination of this surplus material is possibly the means by which the nucleo-cytoplasmic ratio normal for the particular ovum is established, as suggested by G. Levi, but the necessity or its occurrence is, we would emphasize, the outcome of the phylogeny of the Therian ovum, i.e. of the fact that it has been derived from such a meroblastic yolk-laden ovum as is found in the existing Monotremes and Eeptiles, and is not to be regarded simply as one of the ' manifestations dynamiques de la fecondation ' as Brachet (16) would seem to suggest, though the act of fertilization may set the process in operation just as it does cleavage.
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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. (2020, April 9) Embryology The early development of the cat 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/The_early_development_of_the_cat_2
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