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 PKOCESS of CLEAVAGE.

1. DESCRIPTION OF CLEA V AGE. 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 unfor- tunately 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.

Egg 12.

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 peri- pherally, 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, coin- cident 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 peri- pheral 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, excen- trically 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 impor- tance 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 hori- zontal (equatorial), since in the four-celled stage the blasto- meres 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 con- siderably larger than those of the other pair, pointing to inequality in the two-celled stage, and the facts that blasto- mere 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.

Seven-celled Eggs.

We have available for description two seven-celled eggs.

Egg 18 (13.5.19) and egg 19 (17.31.1.13). 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 blasto- mere 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. Blasto- mere 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 :

I	II	III	IV
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 (17.31.1.13). 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. Blasto- mere 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 :

I	II	III	IV
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 blasto- meres 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).

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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

Reference

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

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Cite this page: Hill, M.A. (2024, May 22) 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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