The early development of the cat 1

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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 I.—The Ovum of the Cat

1. Maturation And Ovulation

We give here a short resume of the observations of E. van der Stricht (55) and Longley (37) on the phenomena of maturation and ovulation, supplemented by our own. B. van der Stricht states that oestrus in cats generally lasts from two to three days (? after the first copulation, Longley). At the beginning of oestrus (first day), as a general rule, the intra-ovarian ovum (primary oocyte) has completed its growth, i.e. it has attained the stage of the full-grown ovarian ovum with a peripherally situated germinal vesicle. During the second clay (beginning or end) maturation sets in, and in the course of some hours the first polar body is extruded and the second polar mitotic figure established (stage of the mature ovarian ovum or secondary oocyte). Provided copulation has previously taken place ovulation now follows, and the ovum, still surrounded by the corona radiata of the discus proligerus (which persists for a variable period), is received into the Fallopian tube and is there fertilized, the time of fertilization practically coinciding with the end of oestrus (end of second or third day). Longley states that of a series of ten females killed at periods ranging from twenty-three to fifty hours after pairing, six had ovulated. The ova are said to pass very rapidly through the segment of the tube succeeding the ostium abdominale into its proximal third next the uterus, where, according to B. van der Stricht, fertilization usually appears to be effected though there are exceptions. B. van der Stricht records obtaining a three-celled egg from the middle third of the tube, and Bonnet a nine-celled egg from the same region as well as an egg with two pronuclei, about 1-5 cm. from the ostiuni uterinum. Cleavage, as our own observations show, is completed in the uterine segment of the tube, and the eggs do not pass into the uterus until about the stage when the morula is becoming converted into the early blastocyst through the appearance of the beginnings of the blastocyst cavity. As in other mammals, consequent on the penetration of the sperm into the ovum, the second meiotic division is completed and the second polar body given off. In the absence of fertiliza- tion, the second polar division is not completed (cf. our eggs 1 and 2, referred to below).

The ovum of the Cat thus agrees with that of the great majority of other mammals in that the first meiotic division is completed and the second initiated whilst it is still enclosed in its Graafian follicle in the ovary. In this respect it differs from the ovum of the Dog, which, as 0. van der Stricht has shown, is shed in the condition of a full-grown ovarian ovum, prior to the completion of the first meiotic division. He thinks that the same holds true for the human oocyte, though A. Thomson (56) believes he has been able to observe both meiotic divisions and the elimination of both polar bodies in the intra-ovarian ovum. We agree with 0. van der Stricht that the observations of Thomson stand in need of confirmation.

It appears to be well established that ovulation in the Cat is normally induced by copulation (de Winiwarter and Sain- mont (58), Ancel and Bouin (2), Longley (37), E. van der Stricht (55)). Nevertheless it would occasionally seem to happen spontaneously. Bonnet (14) records finding an unfer- tilized egg in the tube ' bei einer langere Zeit in Einzelhaft gehaltenen und nicht belegten Katze '. We ourselves have obtained from each of two cats a single tubal egg (eggs 1 and 2) unfertilized and devoid of any trace of sperms. In egg 1, the first polar body is distinguishable though its limits are not very clearly defined, and 0028 mm. distant from it there is present in the egg-cytoplasm a group of chromosomes belonging to the second polar mitotic figure, the spindle-fibres of which are not visible. In egg 2 the first polar body has apparently divided. E. van der Stricht (55) records and figures the division of the first polar body in a ripe ovarian egg, and we record it as probable in our eggs 2 and 4. Longley (37) states that division of the first polar body ' is not of very common occurrence, especially in normal eggs '. The second polar mitotic figure is represented by an equatorial plate of small granular chromosomes, situated in the peripheral cyto- plasm 0-05 mm. distant from the first polar body. No spindle- fibres are visible, but from the position of the equatorial plate it is evident that in this case the axis of the second polar figure lies tangentially to the surface and not at right angles thereto as is usual. No doubt these eggs are somewhat abnormal, having probably been some little time in the tubes before fixation, but they serve to demonstrate that spontaneous ovulation can occur in the absence of copulation and that when fertilization is not effected the second polar body is not separated.

2. Structure of the Ovum

The unsegmented tubal egg of the Cat appears in section not quite spherical but ovalish in outline. It varies in sectional diameter from 0084 to 011 x0-09 mm. (average of 0095 x 0082 mm. in thirteen eggs measured). The ovum itself (exclusive of the zona) varies in section from 0-069 x 0-57 mm. to 0-09 x 0-08 mm. in diameter (average of 0-0S2 x 0-07 mm.).

Longley, with reference to the ovarian ovum of the Cat, states that ' the liAang cat's egg has a diameter ranging from 0-135 to 0-15 mm. and is surrounded by a zona 0012 to 0015 mm. in thickness '. We have no measurements either of full-grown ovarian or tubal ova in the fresh state^ but we have records of two tubal eggs which, measured in the fixing fluid, had diameters of 0153 mm. and 0-136 mm. respectively, whilst four eggs, in Hermann's fluid, measured 0-119 x 0-11 mm. in diameter, so that there is actually a considerable variation in the size of the ova. The zona varies in thickness in unseg- mented ova from 00018 to 0006 mm. It increases in thickness during the sojourn of the egg in the tube, and reaches its maximum (0-012 mm.) in late morula and early blastocyst stages which have recently passed into the uterus. It remains intact, though of course greatly thinned out, until long after the blastocyst has become didermic.

Robinson (43) states that in the Ferret, ' the zona pellucida persists until shortly after the formation of the primitive streak in the embryonic area, but it disappears in the region of the trophoblast at an earlier period'. In the Dog he states it ' does not disappear till after the formation of the primitive streak '.

As already noted, the cells of the corona radiata remain adherent to it for a varying period after ovulation, and even after the cells have been lost the basal syncytial layer of the corona seems to persist outside the zona proper, but we are uncertain whether it is eventually lost or is incorporated in the zona. However that may be, when the zona has attained its maximum thickness it appears as a clear, perfectly homogeneous and resistant membrane, which no doubt plays an important r61e in the mechanism of development, since, besides acting as a protection and support for the segmenting egg, it may conceivably act the part of a semipermeable membrane and thus prevent the too rapid absorption of fluid and consequent too rapid expansion of the early blastocyst.

The structure of the ovum has been described in detail by E. van der Stricht, our own observations being largely confirmatory of his. The ovum of the Cat is provided with a considerable amount of deutoplasm mainly in the form of fat-globules of variable size, large, medium, and small. They are less abundant than in the egg of the Dog where they are of medium size, but more numerous than in the ovum of Cavia where they are small (0. van der Stricht, 52).

In the full-grown ovarian ovum, with a peripheral germinal vesicle, E. van der Stricht shows that there is present, as in the preceding growth-stages, a superficial, thin zone of cyto- plasm especially rich in mitochondria, ' une zone corticale mitochondriale plastique (de 0. van der Stricht) ', situated immediately below the egg-membrane and devoid of fat- globules. Within this investing zone, the cytoplasmic body of the ovum contains more or less abundant fat-globules, with mitochondria distributed in the interspaces between them. He distinguishes two groups of eggs according to the amount and disposition of the fat-globules. The first group comprises eggs, relatively rich in fat, the globules being mainly aggregated in the central region and towards one pole, so that the egg possesses a definite polarity, the plastic pole being less rich in fat than the deutoplasmic. In such eggs there is often present, below the superficial layer, a relatively broad zone in which are distributed small fat-globules. The second group com- prises eggs in which the fat-globules are relatively less abun- dant and are mainly grouped in the central region, between which and the superficial layer is a broad zone usually poor in fat except at its periphery, where numerous small globules are generally present. At this stage there is in this group of eggs no obvious polarity. Nevertheless in eggs of both groups during maturation and fertilization stages, polarity, in the distribu- tion of the fat-globules is perfectly definite, though it bears no constant relation to the place of expulsion of the polar bodies. These latter may lie adjacent to each other at the plastic pole as in our egg 9, or at the deutoplasmic as in egg 4, or they may occupy an intermediate position between the two as in egg 5, or again they may lie remote from each other on opposite surfaces of the egg as in egg 7. This variable position of the polar bodies is largely determined by the position originally taken up by the germinal vesicle in the full-grown ovum, that position bearing no constant relation to the deutoplasmic accumulation within the egg (Longley).

In view of the well-known experimental work of Eusso (44) on the ova of the Babbit, E. van der Stricht is inclined to believe that eggs of the first group are destined to produce females, those of the second males, but as to that we offer no opinion. Our own observations on eggs at the stage of fertilization are confirmatory of those of R. van der Stricht so far as concerns the polar distribution of the fat-globules, i. e. in any given egg it is possible, usually without any great difficulty, to satisfy oneself that one hemisphere is richer in fat than the other, but we should hesitate to accept his grouping of the eggs into two clear-cut sets. We agree that some eggs have relatively little fat and others much more, but these, we hold, are but the extremes of a variable series.

Lastly, in connexion with the structure of the ovum, it remains to be mentioned that E. van der Stricht has described the presence in the periphery of the ovum of one or two rounded or ovalish bodies of a size comparable with that of a mammalian red blood corpuscle, and with a structure recalling that of a typical ' vitelline body ' and which he has termed ' corps enigmatique '. They are formed, he says, at the expense of small safraninophil granules that appear in the young oocyte. He regards them as in some way taking part in the formation of the ' plastic vitellus ', and as being also perhaps of the nature of germ-cell determinants. We have also encountered this enigmatical body in our material and record its possible presence in two of the central cells of our sixty-three- celled cleavage stage, but we have not been able to follow it into later stages and are unable to offer any suggestion as to its significance.

3. Fertilization

Comprised in our material are eleven eggs derived from five cats : Cat I, eggs 1 and 2 ; Cat II, eggs (3) and 4 ; Cat III, eggs 5, 6, 7, and (8) ; Cat IV, eggs 9 and 10 ; Cat V, egg 11. These eggs fall into three groups :

• Group 1. Differentiation of pronuclei - egg 1.
• Group 2. Pronuclei remote from each other - eggs (3) and 6.
• Group 3. Pronuclei approximated or in contact - eggs 2, 5. 11, 4, 7, 9, 10, (8). Eggs 3 and 8 contain three pronuclei.

Group 1.

Egg 1 (A. 4.4.17 egg A). Diameter, 0-091 x 0-088 mm.

Ovum, 0084 x 0-081 mm. Zona, 0-002 mm. in thickness, surrounded by cells of corona radiata. Pig. 1, PI. 24. This egg is worthy of notice since it shows an interesting phase in the differentiation of the 3 pronucleus. As may be seen in fig. 1, PL 24, the £ pronucleus lies shortly below the superficial mitochondrial zone and has reached the condition of a membranate vesicular nucleus, but is still small (0-008 x 0-007 mm.) with a shape very much like that of a short blunt- nosed bullet. The chromatin is massed like a cap in its blunt apex, and appears to be in process of differentiation to form a spireme thread. Behind the cap are two ovoidal granules in contact with each other, whilst the remainder of the nuclear space is occupied by a pale-staining reticulum. In contact with the apex of the pronucleus is a granular area of cytoplasm, free from fat-globules, which is possibly the sperm-sphere, but it has not been possible to detect the sperm-centriole. Unfor- tunately the section which contained the main body of the $ pronucleus is the only one missing in the series, but the position of the pronucleus is recognizable in the succeeding section shortly below the second (?) polar body which occupies a distinct bay in the surface of the ovum, adjacent to the deutoplasmic pole. In close proximity to the polar body are several sperm-heads which seem to have penetrated the zona and are therefore supernumerary, but they are similar to those which are found imbedded in the latter. This egg shows particularly well the superficial mitochondrial zone of the egg-cytoplasm described by R. van der Stricht and the mitochondria which are distributed in the cytoplasmic framework supporting the fat-globules. Fat-globules, some of them reaching a large size, are present in abundance throughout the body of the ovum, with the exception of an area extending through three or four sections on one side marking the plastic pole. There is no intermediate zone intervening between the mitochondrial zone and the more internal fat-laden region, the latter abutting directly on the former.

Group 2. Egg 6 (B. 14.17.5.12). Diameter, 0-1x009 mm. Ovum, 009 x0-08 mm. Zona, 00048 mm. thick, homogeneous with numbers of sperm-heads imbedded in it. Figs. 5 and 6, PL 25.

This egg (fig. 5) corresponds closely with egg 94 described and figured by E. van der Strioht (p. 452 and figs. 78-82). The two pronuclei, appearing as large clear spheres, are fully consti- tuted and are indistinguishable except for a very slight difference in size. They occur towards one side of the egg, both appearing in one section (fig. 5), and are separated from each other by an area of cytoplasm 0026 mm. in width in which are present numerous fat-globules. On their outer sides the pronuclei abut on the peripheral plastic zone of cytoplasm containing fine fat-globules in no great numbers. The fat-globules in the body of the ovum are abundant and are mainly massed in the central region and towards one pole (fig. 6), whilst they are sparse and small in the peripheral plastic zone in which there are also present small light-staining homogeneous masses. Polarity in respect of the distribution of the fat-globules is unmistakable. The polar bodies are not distinct, but the ' enigmatical body ' is clearly seen in fig. 5 lying close below the zona and enclosing a central granule. In this egg, as in egg 94 of E. van der Stricht, the central plastic zone in which the pronuclei are eventually found situated, is not yet differen- tiated. Its formation must involve the shifting of the centrally situated fat-globules to a more peripheral position. It is further worthy of note that the pronuclei, as in egg 43 of E. van der Stricht (fig. 43, PI. xv), are so placed that a line joining their centres is approximately at right angles to the polar axis of the egg. Later on, when they have acquired their definitive position in the central plastic region, they lie in that axis, instead of at right angles to it.

Group 3.

Egg 2 (A. 4.4.17 B).. Diameter, 0-091 x 0-076 mm. Ovum, 0-087x0-073 mm. Zona thin, 00018 mm., with the remains of the corona radiata.

The two pronuclei are situated in the less fat-rich hemisphere some distance below the egg-surface. They differ somewhat in size, structure, and position. The smaller of the two is oval, lies towards the centre of the egg, and is, we consider, the ? pronucleus. It measures 0012 x 0009 mm. in diameter

and possesses a subcentral spherical quarrelsome between which and the nuclear membrane is a chromatin-reticulum or rather a spireme thread segmented into five or six pieces which are roughly centred on the karyosome internally and spread out at their peripheral ends on the inner surface of the nuclear membrane. The larger of the two pronuclei (the 3) is spherical and has a diameter of 0-014 mm. It is rather less deeply situated than the smaller, and is separated from it by a distance of 0-016 mm. It contains one large and two smaller karyosomes, and in close relation to the inner surface of the nuclear mem- brane, on the side next the smaller pronucleus, are numbers of small chromatin granules and one larger mass. Immediately adjacent to this pronucleus there occurs a minute ovalish body, 0-008 x 0-002 mm. in diameter, containing a central granule. This body we regard as the sperm-centrosphere. As in the companion egg 1, the cytoplasmic structure is well preserved. The superficial mitochondrial zone is less marked than in egg 1, but the mitochondrial formations in the remainder of the cytoplasm are even more abundant. The fat-globules are numerous and of large size and are mainly located in one hemisphere, the pronuclei being situated in the other. The polar bodies, situated close together, are seen in surface view : one is larger and possesses two chromatin clumps, the other appears to be divided into two unequal parts.

Egg 5 (14.17.5.12 A).

Diameter, 0-1 x 0-084 mm. Ovum, 0086x0072 mm. Zona, 0-004 mm., with numbers of sperm-heads imbedded in it. Fig. 7, PI. 25.

The pronuclei are approximated but not in actual contact. They are large and vesicular, with one or two karyosomes and a distinct linin reticulum with scanty chromatin. Of the two, one (fig. 7) is smaller (0-021 mm. in diameter) and situated nearer the polar bodies ; we regard it as the ? pronucleus ; the other (g) is larger (0028 mm.) and rather more superficially situated. Both lie in the plastic hemisphere. The fat-globules (of medium to small size) are larger and more abundant towards one pole (fig. 7) ; towards the opposite or plastic

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The fat-globules are small, fairly numerous, and mostly aggregated in one hemisphere, leaving the other 'hemisphere in which the pronuclei are situated relatively free. The two polar bodies lie adjacent to each other at the plastic pole.

Egg 10 (15.23.5.12 B).

Diameter, 0-088 x 0-069 mm. Ovum, 0079x0057 mm. Zona, 00036 mm. Kg. 9, PI. 25.

It is noteworthy that eggs 9 and 10 from Cat IV are both small. The two pronuclei lie in contact in the central region of the egg. They differ in size. The smaller (the ?) is approxi- mately central and is spherical, with a large central and one or two smaller karyosomes and peripheral chromatin granules. Immediately outside the nuclear membrane is a very distinct centrosome granule. The larger pronucleus (the d) is ovalish and is situated nearer the plastic pole. It possesses several karyosomes and chromatin masses located mainly on the side nearest the smaller pronucleus. The nuclear membrane appears indistinct on the side nearest the plastic pole. There is an unmistakable polarity in the disposition of the fat-globules. They are less numerous, but many of them reach a much larger size than in egg 9, and they are mainly located in one hemi- sphere, though they extend up around the central region in which the pronuclei are situated. Plastic and deutoplasmic poles are readily recognizable, the region of the plastic pole being almost free from fat. The polar bodies and ' corps enigmatique ' were not observed.

Egg 4 (13.17.5.12 A).

Diameter in fixative, 0-102 x 0-11 mm.; in section, 009x0089 mm. Ovum, 0069x0057 mm. Zona with remains of corona radiata attached and with sperm-heads imbedded in it.

The ovum has contracted away from the zona, leaving a large penivitelline space.

The two pronuclei lie in contact in the central region of the ovum which is free from fat-globules. They are large and vesicular and differ slightly in size. The larger one is ovalish in form and possesses a coarse reticulum with one large and numerous smaller karyosomes which tend to be collected on the side nearest the other pronucleus. The latter, slightly the smaller of the two, is spherical and possesses a coarse reticulum with numbers of small karyosomes, again tending to be grouped on the side nearest the other pronucleus. The fat-globules are small and not very numerous and are most concentrated towards one pole, that adjacent to which are situated the polar bodies. The latter lie in proximity to one another and comprise a smaller one with a trilobate clump of chromatin and a larger, possibly divided into two.

Egg 11 (9.5.19).

Diameter, 0 0 9 x 0 0 7 6 mm. Ovum, 0084x0069 mm. Zona, 0-003 mm., with sperm-heads imbedded in it. Fig. 2, PI. 24.

This well-fixed egg is at the stage just preceding the forma- tion of the first cleavage spindle. The egg-cytoplasm consists of a very narrow finely granular and more deeply staining superficial zone, which is thicker at the plastic pole than else- where, and a coarsely granular ground-mass in which the fat- globules are situated. The fat-globules, which are not very abundant, are mainly grouped towards one pole of the egg, below and especially round the pronuclei which are subcentral in position. The pronuclei are approximated but not in actual contact and are of unequal size, the larger measuring 0-024 x 0-021 mm., and the smaller 0-018 x 0-016 mm. in diameter. The larger pronucleus (the <J) shows a delicate nuclear membrane, a coarse reticulum, three larger karyosomes in contact and one smaller one and a group of chromosomes situated on the side of the nucleus nearest to the other, many of the chromosomes lying in contact with the nuclear membrane. The smaller pronucleus (the ?) has essentially the same structure ; it possesses a single karyosome, a coarse reticulum and well- marked chromosomes, mostly situated in close contact with the nuclear membrane on the side nearest the larger pro- nucleus. We have found it impossible to make an accurate count of the chromosomes. It seems evident from this egg that the chromosome groups from the two pronuclei will pass directly into the equator of the first cleavage spindle, without the formation of a cleavage nucleus. We have not been able to distinguish the sperm-centrosphere or its derivatives. In the section figured (fig. 2) the two pronuclei are separated by an irregular clear space which is possibly an artefact, whilst to one side of the large pronucleus is an irregular deeply staining body which we at first thought might have been dragged out of the pronucleus during the sectioning of the egg, but the reticulurn appears undisturbed and the nuclear membrane intact. Its significance must remain problematical.

Two polar bodies are present, of unequal size and situated in the deutoplasmic hemisphere, the smaller one not far from the deutoplasmic pole. The larger one (in all probability the first) has a diameter of 0012 x 0-005 mm.; it lies in a bay at the surface and contains eight irregular masses of chromatin. The smaller one measures 0009 x 0-003 in diameter and con- tains a single irregularly spherical mass of chromatin.

Amongst our fertilization stages we have met with two eggs in each of which there is a supernumerary male pronucleus, as in egg 96 of B. van der Stricht.

Egg 8 (D. 14.17.5.12).

Diameter, 0-1 mm. Ovum, 0-084 x 0-079 mm. Zona (exclusive of outer follicular investment), 00036 mm.

There are two pronuclei, a larger (0-021 x 0-019 mm.) and a smaller (0-016 x 0-014 mm.), which lie in contact towards one side of the ovum, and which correspond to the pronuclei of the normal egg, whilst a supernumerary pronucleus is situated shortly below the surface at the opposite side of the egg and is of practically the same size as the larger one of the other pair. The fat-globules are most abundant in the region of the egg between the accessory and the paired pronuclei, and round the latter are also fairly numerous whilst they are less numerous in the region occupied by the supernumerary pronucleus, as in egg 96 of E. van der Stricht. Polarity in the distribution of the fat-globules is therefore indicated, the pair of pronuclei lying in the more deutoplasmic region. The polar bodies are apparently normal.

Egg 3 (13.17.5.12 B).

Diameter in fixative, 0-10 mm., enclosed by remains of corona radiata in which are imbedded numbers of sperm-heads.

This egg is very similar to the preceding. There are two pronuclei, a larger and a smaller lying in contact shortly below the surface on one side and a supernumerary one of about the same size as the larger one of the pair but less rich in chromatin, situated superficially at the opposite side of the egg. The two polar bodies lie in contact, and possess one and two angular masses of chromatin respectively, the one with two chromatin masses having a diameter of 0009 x 0007 mm.

The special interest of these eggs is that they show that the larger of the two pronuclei in normal eggs is the male. This is in agreement with the statement of Lams (36) that in Cavia the male pronucleus is always the more peripheral and is always larger than the female pronucleus.

4. Remarks on Fertilization

Our knowledge of the phenomena of maturation and fertilization in the monodelphian mammals is now extensive, thanks to the work of O. van der Stricht (50, 52), H. Lams (35, 36), Sobotta (47), and others. 0 . van der Stricht (52) has recently given a most valuable r e s u m e of his own investigations and those of his pupils, to which we would refer the reader.

Our observations on the fertilization process in the Cat are in the main in agreement with those of R. van der Stricht. Contrary to his statement, however, we are able to demonstrate that the pronuclei, once they have reached their definitive condition, are not identical but differ in size and in position, the cj pronucleus being always larger and always more super- ficially situated, nearer the upper or plastic pole, than the ? pronucleus. This is in agreement with the conclusion of Lams (36) for the pronuclei in Cavia as above stated, and of O. van der Stricht (50) for those of Vesperugo.

The pronuclei, after they have enlarged and assumed their characteristic vesicular form, migrate inwards and come to lie in the central plastic region of the egg which is more or less free from large fat-globules ; if fat-globules are originally present in this central region as in our egg 6 and egg 94 of R. van der Stricht, they would seem to undergo displacement towards the deutoplasmic pole, but apart from this they main- tain their original polar distribution during the fertilization process, no reversal of polarity such as 0. van der Stricht (50) and Lams (36) have described for Vesperugo and Cavia occur- ring in the Cat. The pronuclei, once they have reached the central plastic region, come to lie in close proximity or in actual contact, and acquire a disposition such that the larger (<$) pronucleus is situated nearer the plastic pole, the smaller (?) pronucleus nearer the deutoplasmic pole. The line joining their centres marks, as R. van der Stricht has emphasized, the definitive axis of the egg and the plane of the first cleavage, and it coincides as near as may be with the polar axis. Although the egg of the Cat presents an unmistakable polarity owing to the greater abundance of fat-globules at the lower pole as compared with the upper, there is a considerable amount. of variation and irregularity in their distribution in different eggs, with the result that it is often difficult to determine precisely the position of the polar axis, but once the pronuclei have acquired their final position the definitive egg-axis is readily determinable.

Like R. van der Stricht, we have never been able to observe the tail of the spermatozoon in the egg-cytoplasm. In both the Cat and the Dog, as 0. van der Stricht points out, the sperm-tail is very delicate and often difficult to see outside the ovum ; he records, however, that he has been able to demon- strate its presence in several ova of the dog.

Only in one ovum (egg 2) have we been able to observe the sperm-centrosphere, whilst in another (egg 10) we record the presence of what we regard as the egg-centrosonie (centriole) adjacent to the ? pronucleus. The observations of 0. van der Stricht (50) and Lams (36) on the process of fertilization in Vesperugo and Cavia, where the sperm-tail remains adherent to the central corpuscle of one of the poles of the first cleavage spindle demonstrate that in these mammals ' le ou les sper- matocentres persistent et participent a l'edification des deux spheres attractives definitives de la premiere etoile-mere de l'oeuf en division ' (52, p. 15).

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, April 18) Embryology The early development of the cat 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/The_early_development_of_the_cat_1

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