Paper - Contributions to the embryology of the marsupialia 4-3
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Hill JP. The Early Development of the Marsupialia, with Special Reference to the Native Cat (Dasyurus Viverrinus). (1910) Quart. J. Micro. Sci. 56(1): 1-134.
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Chapter III. - Cleavage and Formation op the Blastocyst
1. Cleavage
CCleavage begins in the uterus as in Didelphys, Phascolarctus, and no doubt Marsupials in general. The first externally visible step towards it consists, as already described, in the elimination by abstriction of the deutoplasmic zone at the upper pole. The yolk-body so formed appears as a definitely limited, clear, rounded mass which lies in contact with the slightly concave upper surface of the formative remainder of the ovum. It is quite colourless and transparent except for the frequent occurrence in it of a small, more or less irregular opaque mass, representing probably a condensation product of its fluid material (cf. PI. , figs. 8, 14, y.h.). Consisting as it does of a very delicate cytoplasmic reticulum with fluid-filled meshes it is extremely fragile, and is seen to advantage only in fresh material (figs. 14 and 19, y.h.). It takes no direct part in the later developmental processes> though during the formation of the blastocyst it becomes enclosed in the blastocyst cavity and finally undergoes disintegration therein, its substance becoming added to the fluid which Alls the same, so that it may be said, in this indirect way, to fulfil, after all, its original nutritional destiny. Separa
THE EARLY DEVELOPMENT OP THE MARSUPIALIA. 29
tion of the yolk-body is rapidly followed by the completion of the division of the formative remainder of the ovum into the first two blastomeres, the plane of division being coincident with the polar diameter or egg-axis and at right angles to the plane of separation of the yolk-body (PL 2, fig. 14). I obtained relatively little material between the stage of the unsegmented ovum with two equal-sized pronuclei seen in fig. 12 and the 2-celled stage (fig. 14), both of which are well represented in my material, so that it would appear that the separation of the yolk-body and the division of the formative remainder of the ovum are effected with considerable rapidity. Fig. 13 shows, however, a section of an unsegmented ovum in which the chromosomes of the metaphase of the first cleavage figure are visible in the central region of the formative cytoplasm, but situated, it is worthy of note, rearer the future upper pole than the lower pole. The deutoplasmic zone {d.z.) still forms an integral part of the egg, and there is no sign of commencing abstriction. I have also sections of ova in a still more advanced stage of the first cleavage, in which the daughter-nuclei have but recently been constituted and are still quite minute, and the cleavage furrow is well marked on the surface of the egg. In these ova the yolk-body is already separated, so that we may conclude with a fair degree of certainty that its elimination about coincides with the first appearance of the cleavage furrow.
Figs. 14-16 show the 2-celled stage, respectively in side, lower polar, and end views. The blastomeres are of approximately equal size and otherwise quite similar. Selenka also found the same to be the case in Didelphys, though in the single specimen of the 2-celled stage he had for examination (Taf. xvii, fig. 3) the blastomeres are displaced and somewhat shrunken. Each blastomere has much the shape of a hemisphere from which a wedge-shaped segment has been sliced off, a form readily accounted for when we take account of the effect of the elimination of the deutoplasmic zone. After that event, the formative remainder of the ovum has the form of a sphere from which a somewhat bi-convex lens
30
J. P. HILL.
shaped piece lias been gouged out at the upper pole. Consequently, when it divides along its polar diameter, the resulting blastomeres will have the form of hemispheres with obliquely truncated upper surfaces or ends, which will be proportionately thicker than the lower ends. In correlation therewith we find the nucleus of each blastomere situated slightly excentrically, rather nearer the upper than the lower pole (fig. 18). The rounded yolk-body lies partly enclosed betAveen the upper truncated surfaces of the blastomeres.
Two-celled eggs ai'e shown in vertical section in figs. 17 and 18. The cytoplasm of the blastomeres exhibits a wellmarked differentiation into two zones corresponding to that ah'eady seen in the formative cytoplasm of the uusegmented egg, only much more accentuated, viz. a dense, fine-grained perinuclear zone, and a less dense, more vacuolated peripheral zone, in which there is present a coarse, irregular network of deeply staining strands, recalling the frameAvox'k of mitochondrial origin described by Van der Stricht ('04, '05) in the human ovum and that of Vesperugo. We have here in this differentiation of the cytoplasm, evidence of the occurrence of an intense metabolic activity Avhich has resulted in a marked increase in the amount of deutoplasmic material present in the blastomeres as compared Avith that found in the ovarian egg or even in the unsegmented uterine egg. The blastomeres consequently present a someAvhat dense opaque appearance Avhen examined in the fresh state, their nuclei being partially obscured from view. Amongst the Eutheria, various observers (Sobotta, Van der Stricht, Lams and Doorme) have described a similar inci'ease in the deutoplasmic contents of the egg after its passage into the Fallopian tube or uterus.
The second cleavage plane is also vertical and at right angles to the first. The resulting four equal-sized blastomeres vieAved from the side (PI. 2, fig. 19) are seen to be ovalish in outline, their loAver ends being slightly narroAver and more pointed than their upper ends, Avhich diverge someAvhat to enclose the lower part of the yolk-body. Seen from one of
THE EAELY DEVELOPMENT OF THE MARSUPIALIA.
31
tli 0 poles, in optical section (figs. 20, 21), they appeal triangular 'with rounded corners and centrally directed apices. The space occupying the polar diameter, which they enclose is the cleavage cavity. The blastomeres are now somewhat less opaque than those of the 2-celled stage, so that their nuclei, excentrically situated nearer their upper ends and enclosed in the central granular zone of the cytoplasm, can now be fairly distinctly made out in the fresh egg.
The arrangement of the blastomeres at this stage is exceedingly characteristic, and is identical with that of the blastomeres in the corresponding stage of Amphioxus or the frog, but is quite different from that normal for the 4-celled stage of the Eutheria. They lie disposed radially or meridionally around the polar diameter, occupied by the cleavage cavity, their thicker upper ends partially surrounding the yolk-body. Selenka figures a precisely similar arrangement in his 4-celled stage of Didelphys, so that we may conclude it holds good for the Marsupials in general.
Whilst, then, in Marsupials the first two cleavage planes are vertical or meridional, and at right angles to each other, and the first four blastomeres are arranged radially around the polar diameter (radial type of cleavage), in the Eutheria such is never the case, at all events normally, so far as is known. In the Eutheria the first four blastomeres form, or tend to form, a definite cross-shaped group, as the result apparently of the independent division of thefii'st two blastomeres in two different planes at right angles to each other, the division planes being meridional in the one, equatorial in the other. ^ This pronounced diffei'ence in the spatial relations of the fij'st four blastomeres in the Metatheria and Eutheria is a feature of the very greatest interest and importance, since it is correlated with and in part conditions the marked dissimilarity which we meet with in the later developmental occurrences in the two groups, in particular in the mode of formation of the blastocyst in the two.
Compare in this connection Assheton's remarks ('09, pp. 232-233), which have api>eared since this chapter was written.
32
J. P. HILL.
Moreover, so far as the Eutheria are concerned, it affords us, I believe, a striking and hitherto unrecognised example of a phenomenon to which Lillie ('99) has directed attention, viz. adaptation in cleavage.
Fig. 22 shows a horizontal section through the 4-celled stage, and fig. 23 a vertical section of the same. The blastomeres in their cytoplasmic characters essentially resemble those oE the 2-celled stage, but the peripheral deutoplasmic network is here more strongly developed, and it is especially worthy of note that it is more marked towards the lower poles of the blastomeres (fig. 23), as also appears to be the case in the 2-celled stage. The shell-membi'ane measures in thickness '0072 mm.
The next succeeding (third) cleavages are again meridional, each of the four blastomeres becoming subdivided vertically into two, not necessarily synchronously. Fig. 53. PI. 6, shows a side view, and fig. 54 a view from the lower pole of a 6-celled egg, two of the blastomeres of the 4-celled stage having divided before the other two. The blastomeres have moved apart, and now form an' open ring approximately equatorial in position, and surrounding the central cleavage space, the upper opening of which is occupied by the yolk-body. I have failed to obtain a perfectly nonnal 8-celled stage, nevertheless the evidence clearly shows that the first three cleavage generations in Dasyurus are meridional and equal, and that the resulting eight equal-sized blastomeres form an equatorial ring in contact with the inner surface of the sphere formed by the zona and shell-membrane.
Whilst, then, the first three cleavage generations are meridional and equal, the succeeding divisions (fourth cleavage generation), on the contrary, are equatorial and unequal, each of the eight blastomeres becoming divided into a smaller, more transparent upper cell, with relatively little deutoplasm, and a larger, more opaque lower cell with more abundant deutoplasmic contents. In this way there is formed an exceedingly characteristic 1 6-celled stage, consisting of two superimposed rings, each of eight cells. The upper ring of smaller and clearer cells partially encloses the yolk body, and is situated entirely in the upper hemisphere of the sphere formed by. the egg-envelopes. The lower ring of larger, more opaque cells lies approximately in the equatorial region of the said sphere. This 16-cell ed stage is.figm'ed in fig. 55, PI. 6, as seen from the side, and in fi'g. 56 as seen from the upper ^ pole, both figures being taken from a spirit egg '37 mm. in diameter. The marked' differences in the cells of the two rings are well brought out in the micro-photographs reproduced as figs. 24, 25, and 26, PI. â– 2. Pigs. 24 and 25 represent horizontal sections of an egg '38 mm. in diameter, the former showing the eight cells of the lower ring, and the latter the eight cells of the upper ring. Pig. 26 shows a vertical section through an egg also of a diameter of "38 mm., but with seventeen cells, one of the original eight cells of the upper ring having divided and one beinginprocessof division. Thesection passes through the yolk-body [y.h.), which is seen as a faintly outlined structure lying in contact with the zona between the two cells of the upper ring (/.c.).
The shell-membrane in eggs of this 16-celled stage has attained a thickness of -0075 mm., and the albumen layer has been almost completely absorbed, so that the zona now lies practically in apposition with the shell-membrane, the two together forming a firm resistant sphere, to the inner surface of which the blastomeres are closely applied. The separation between the zona and shell-membrane seen in the figures is largely, if not wholly, artificial. ,
The average^ measui'ements of the cells of the two rings ip the '38 mm. egg, :^ured in figs. 24 and 25, are as follows :
Ppper ring cells. Lower ring cells.'
Diameter . : -06 x -058 mm. . -OOx-Ofidinm.^,'
Vertical height , -OSSimm. . -llS mm.
Nucleus . j . ‘0165 mm. ■■• . •02 mm.
These measurements demonstrate at a glance the distinct difference in. size which exists between the cells of the two rings, whilst the cytoplasmic differences between them. are equally evident from an inspection of the micro-photographs, figs. 24-26. In the larger cells of the lower ring (fig. 24, tr.ect.) the nucleus (rich in chromatin and nucleolated) is surrounded by a perinuclear zone of clearer, coarsely vacuolar cytoplasm, outside of which is a densely granular deutoplasmic zone, which extends to within a short distance of the periphery of the cell-body. In the smaller cells of the upper ring (fig. 25, /.c.) the cytoplasm is coarsely reticular, with a tendency to compactness round the nucleus, and its contained deutoplasmic material is spare in amount as compared with that of the lower cells, being mainly located in a quite narrow peripheral zone. The upper cells thus appear relatively clear as compared with the dense, opaque-looking lower cells (fig. 26).
It becomes evident, then, that we have to do here, in this fourth cleavage generation, with an unequal qualitative division of the cytoplasm of the blastomeres of the 8-celled stage. Just such a division as this we should expect if the deutoplasmic material were mainly aggregated towards the lower poles of the dividing cells. The evidence shows that this is actually the case. In the 2-celled and especially in the 4-celled eggs we have already seen that the deutoplasmic network is already most strongly developed towards the lower poles of the blastomeres. This polar concentration of the deutoplasm reaches its maximum in blastomeres of the 8celled stage, and confers on these an obvious polarity. Although I failed to obtain normal examples of the latter stage, I have fortunately been able to observe the characters of the blastomeres in sections of eggs with twelve, thirteen, and fourteen cells respectively.
In the 12-celled egg (PI. 6, fig. 57), measuring '38 mm. in diameter, four of the eight original blastomeres are still undivided ; the remaining four have undergone division unequally and qualitatively, one but recently, so that 4 + (4 X 2) = 12. The undivided blastomeres are large (average diameter, '11 x '076 mm.) and ovoidal in form, their lower ends being thicker than their upper, and they exhibit a well marked polarity. The nucleus lies excentrically in the upper half of the cell, just above the equator, and is surrounded by a finely granular zone of cytoplasm, outside which is a thin irregular ring of deutoplasmic material. The cytoplasm of the apical part of the cell is clear and relatively free from deutoplasm ; that of the lower half, on the other hand, is so rich in deutoplasm as to appear quite dense and opaque. The conclusion is therefore justified that the blastomeres of the 8-celled stage possess a definite polarity, which has beenj acquired as the result of the progressive concentration of deutoplasmic material at their vegetative poles during the> cleavage process. Division, in the equatorial plane, of cells so constituted must necessarily be unequal and qualitative, so far at least as the cytoplasm is concerned.
In the 13-celled stage three of the original eight blastomeres are in process of division, and five have already divided unequally and qualitatively, so that 3 -f- (5 x 2) = 13, and in the 14-celled stage two of the original blastomeres are in division and sis have already divided : 2 -t- (6 x 2) = 14.
The significance to be attached to this characteristic unequal and qualitative division of the blastomeres of the 8-celled stage to form two superimposed cell-rings, markedly differentiated from each other, we shall presently consider. Meantime I may categorically state the conclusions I have reached in regard thereto. The wall of the blastocyst in Dasyurus is at its first origin, and for some considerable time thereafter, unilaminar throughout its entire extent, and I regard the upper cell-ring of the 16-celled stage as giving origin to< the formative or embryonal region of the unilaminar wall, the lower cell-ring ae furnishing the extra-embryonal or nonformative remainder of the same. I shall therefore refer to the upper cell-ring and its derivatives as formative or embryonal, and to the lower, cell-ring and its derivatives as non-formative or extra-embryonal.
The formative or embryonal region furnishes the embryonal ectoderm and the entire entoderm of the vesicle, and I accordingly conclude that it is the homologue of the embryonal knot or inner cell-mass of the EutHerian blastocyst. The nonformative or extra-embryonal region directly gives origin to the outer extra-embryonal layer of the bilaminar blastocyst wall, i.e. to that layer which in the Sauropsida and Prototheria is ordinarily termed the extra-embryonal ectoderm. I regard it as such, and as the homologue of the so-called trophoblast (or as I prefer to term it, the “ trophoblastic ectoderm†or “ tropho-ectoderm â€) of the Eutherian blastocyst.
A word or two here before concluding this section by way of summary, as to the condition of the enclosing egg-envelopes. During the sojourn of the egg in the uterus the albumen is gradually resorbed, and by about the 16-cell stage it has all but completely disappeared, thus permitting the zona to come into direct apposition with the inner surface of the shellmembrane. The shell-membrane itself increases very considerably in thickness during cleavage, and by the 16-celled stage had practibklly reached its maximum, viz. '0075•008 mm., i.e. it is nearly five times thicker than that of the ovum which has just entered the uterus. The thickened shell-membrane by itself is firm and resistant, and it becomes-, still more so by the application-of the zona to its inner surface, the two together forming a spherical supporting case round the segmenting egg, to the inner surface of which the blastomeres become closely applied. .
The existence of such a firm supporting envelope round the Marsupial egg is, in my view, a feature of very great ontogenetic significance, and one which must be taken into account in any comparison of the early developmental occurrences in the Metatheria and Eutheria. As the sequel will show, the mode of formation of the blastocyst in these two sub-claisses is fundamentally different, and in my opinion the explanation of this difference is to be found in the retention by the Metatheria of a relatively thick resistant; shell-membrane, and its complete disappearance amongst the Eutheria.
2. Formation of the Blastocyst
It is characteristic of the Marsupial that the cleavage-cells proceed directly to form the wall of the blastocyst, without the iuterventioii of a morula stage, as in' the Eutheria.
The fil'tli cleavages are meridional, each of the eight cells of the two rings of the 16-celled stage becoming subdivided vertically into two, so that there results a 32^celled stage consisting of two rings, each composed of sixteen cells. As might be expected, the smaller less- yolk-rich cells of the upper ring tend to divide more rapidly than the larger yolkladen cells of the lower ring, but the difference in the rate of division of the two is only slight. I have, for example, sections of a 17-celled stage (that already referred to, hg. 26) consisting of nine formative cells (= 6 + [1 x 2] + 1 in division) and eight non-formative cells, and also of a 31-celled stage (PI. 6, tig. 59, seen from lower pole; cf. also tig. 60, showing a side view of another 31-celled egg, both eggs "375 mm. in diameter), consisting of sixteen formative and fifteen non-formative cells, of -which one is in process of division. But I have also preparations of 32-celled eggs with ail equal number of formative and non-formative cells, showing that the latter may make up their leeway, the former resting meantime. On the other hand, the cells of the two rings may divide more irregularly, as evidenced by a stage of about forty-two cells, consisting approximately of twentythree formative cells ( = 9 -f- [7 x 2J ) and nineteen nonformative (= 13 -f [3 X 2]). Whatever the rate of division, the important point is that the division planes are always radial to the surface, so that all the resulting blastomeres retain a superticial position in contact with the inner surface of the supporting sphere formed by the zona and shell-membrane. In apposition with the continuous surface afforded by that, the blastomeres, continuing to divide, gradually spread round towards the poles, the descendants of the upper or formative cell-ring gradually extending towards the upper pole marked by the yolk-body, whilst those of the lower or uou-formative cell-riug similarly spread towards the lower pole. As the blastomeres divide and spread they become smaller and more flattened, and gradually cohere together, and so in this way they eventually give origin to a complete unilaminar layer lining the inner surface of the sphere formed by the egg-envelopes. It is this unilaminar layer which constitutes the wall of the blastocyst.
The just completed blastocyst of Dasyurus is a spherical fluid-hlled vesicle measuidng about '4 mm. in diameter (PI. 3, flgs. 27-29, PI. 6, figs. 61, 62), and invested externally by the thin zona and the shell-membrane {■0075-0078 mm. in thickness). The albumen layer has completely disappeared, and the shell-membrane, zona, and cellular wall are from without inwards in intimate apposition. The smallest complete vesicles which I have examined measure ‘39 mm. in diameter (figs. 27, 61), and in one of these I find the cellular wall consists approximately of about 108 cells. In four other eggs of the same diameter and from the same female the wall of the blastocyst is as yet incomplete at the lower pole (fig. 31, l.'p.), and in these, rough counts of the cells yielded the following respective numbers - 89, 93, 121, 128. In another also incomplete blastocyst of the same batch, '41 mm. in diameter (fig. 32), the cellular wall consists of about 130 cells. 'I'he largest complete blastocyst in this same batch measured "49 mm. in diameter, so that we have a range of variation in size of the just completed blastocyst extending from ‘39 to '49 mm.
The unilaminar wall of the blastocyst consists of a continuous layer of more or less flattened polygonal cells (figs. 27-29, 61, 62) lying in intimate contact with the zona, itself closely applied to the shell-membrane. Over the lower hemisphere the non-formative cells are on the whole larger and plumper than the formative cells of the upper hemisphere, and in sui-face examination they appear somewhat denser owing to the fact that they possess much more marked perinuclear zones of dense cytoplasm than do the formative cells (cf. fig. 63, representing a ‘6 mm. vesicle). In sections, however, this latter difference is much less obvious, indeed is hardly, if at all, detectable, so that one has to depend partly on the relative thickness of the cells, partly, and, indeed, mainly, on the yolk-body in determining which hemisphere is which.
The blastocyst cavity is tensely filled by a coagulable fluid derived from that poured into the uterine lumen through the secretory activity of the uterine glands. Also situated in the blastocyst cavity, in contact with the inner surface qf the wall in the region of the upper pole, is the spherical yolkbody (fig. 29, y.h.). It becomes overgrown and enclosed in the blastocyst cavity as the result of the completion of the cellular wall over the upper polar region, much in the same sort of way as the yolk in the meroblastic egg becomes enclosed by the peripheral growth of the blastoderm. In the majority of my sections of early blastocysts the yolk-body has been dragged away from contact with the formative cells through the coagulation of the albuminous blastocystic fluid, and lies more or less remote from the wall enclosed by the coagulum, except on the side next the upper hemisphere (fig. 31, y.h., c.g.). In two instances, one of which is shown in fig. 32, 1 find the yolk-body had become so firmly attached to one of the formative cells that the coag-ulum formed during fixation failed to detach it, and only succeeded in drawing it out to a pear-shape.
The yolk-body, it may here be mentioned, persists for a considerable time in the blastocyst cavity; I have found it shrunken indeed, but still recognisable, in relation to the embryonal area in vesicles 4'5-6 mm. in diameter. And there may even appear within it peripherally, irregular strands which stain deeply wit^ iron-h<ematoxylin and which recall those forming the periphei'al deutoplasmic network of the early blastomeres. Eventually, however, it seems to disappear, its substance passing into the blastocystic fluid, so that, as already remarked, it fulfils in this indirect way its original destiny.
Normally the cavity of the just completed blastocyst contains no cellular elements whatever. In one otherwise perfectly normal blastocyst ('39 mm. diam.) I find present however, a small spheroidal body *028 mm. in diameter, composed of glasSy-looking cytoplasm enclosing a central deeply staining granule. This I interpret as a cell or cellfragment which has been accidentally separated off from the wall, and which has undergone degeneration. In later blastocysts such cellular bodies exhibiting more or less evident signs of degeneration are of fairly common occurrence. They are of no morphological significance.
Selenka^s Blastopore - Normally the wall of the blastocyst is first completed over the upper hemisphere, in correspondence with the fact-that the formative cells not only divide somewhat more rapidly than the non-formative but have a smaller extent of surface to cover, since the upper cell-ring from which they are derived lies about midway between the upper pole of the sphere formed by the eggenvelopes and the equator of the same, whilst the lower cellring from which the noil-formative cells arise is approximately equatorial in position. We thus meet with stages in the formation of the blastocystic wall such as are represented in surface view on PI. 3, fig. 30, and in section in figs. 31 and 32, in which the blastocystic cavity, prior to the completion of the cellular walTbver the lower polar region, is more or less widely open below. There can be no doubt, I think, but that this opening corresponds to that observed by Selenka in his 42-celled “gastrula^' of Didelphys and regarded by him as the blastopore, since he believed the entoderm arose from its lips. My observations conclusively show that it has no connection whatever ■with the entoderm, this layer arising from the formative region of the upper hemisphere, and that it is a mere temporary opening of no morphological significance, blastoporic or other. Pi'ior to the completion of the wall at the upper pole a corresponding opening is temporarily present there also. Both owe their existence to the characteristic way in which the blastocyst wall is formed by the spreading of the products of division of the two cell-rings of the 16-celled stage towards opposite poles in contact with the surface provided by the enclosing egg-envelopes.
I have met with one specimen, an incomplete blastocyst •39 mm. in diameter (belonging to the same batch as the other blastocysts referred to in this section^), in which the lower hemisphei'e would appear to have been completed before the upper, for the yolk-body lies in contact with the zona in the region where the cellular wall is as yet absent, and that the yolk-body has not been secondarily displaced is proved by a micro-photograph of the specimen in my possession (taken immediately after its transference to the fixing solution), in which the yolk-body is seen to lie at the unclosed pole in exactly the same position as in the sections.
In connection with this exceptional specimen, it may be recalled that Selenka, in his 68-celled “ gastrula†of Didelphys (fig, 10, Taf. xvii), figures the wall as complete at the lower pole, the “blastopore†having alx'eady closed, but as still incomplete at the upper pole, there being present a small opening leading into the blastocyst cavity. In the 42-celIed “gastrula†(fig. 8, Taf. xvii) this same opening and the “blastopore†as well are present. The occurrence of these openings at opposite poles, and the general agreement in the constitution of the blastocyst wall (larger, more yolk-rich cells at lower pole, smaller, less yolk-rich cells at upper), in the corresponding stages in Didelphys, and Dasyurus justify the conclusion that the blastocyst of the former develops in the same way as does that of the latter. It is worthy of remark, however, that the just completed blastocyst of Didelphys appears to be considerably smaller than that of Dasyurus. Selenka unfortunately gives no measurements of his early stages, but I have calculated from the figure, the magnificatiou of which is given, that the ^8-celled blastocyst has a diameter of about ‘IS? mm. The corresponding stage of Dasyurus measures about '39 mm., and is therefore nearly three times as large.
• This batch, from female 2 b, 16 . vii . '01, comprised altogether twenty-eight eggs, of which some eighteen were normal complete and incomplete blastocysts mm. in diameter) and ten abnormal, four
of these being unsegmented ova.
Selenka's Urentoderinzelle. - Whilst the 42- and 68celled blastocjsts described by Selenka may be regarded as normal so far as the occurrence of polar openings and the constitution of their wall ai‘e concerned, I hold them to be abnormal in respect of the presence in each of a single large yolk-laden cell, regarded by Selenka as entodermal in significance. It is well to point out that Selenka was not able actually to determine the fate of this cellj he merely presumed that it took part in the formation of the definitive entoderm. No such cell occurs in normal blastocysts of Dasyurus at any stage of development, and in my opinion Selenka's “ urentodermzelle†is none other than a retarded and displaced blastomere, i.e, a blastomere which has failed for some reason to divide, and which has become secondarily enclosed by the products of division ot its fellows, and I am strengthened in this interpretation by the occurrence in an abnormal blastocyst of Dasyurus of just such a large cell as that observed by Selenka. The vesicle in question is one of the batch already referred to, and measured '397 mm. in diameter. The cellular wall (fig. 37) is apparently normal, but is incomplete' at one spot, and the gap so left is occupied by a large binucleated cell, i-ich in deutoplasm and measuring •12 X '072 mm. (fig. 37, abn.). This cell corresponds in its size and cytoplasmic characters with a non-formative blastomere of about the 16-celled stage, and I l egard it simply as a blastomere which has failed to undergo normal division. In another abnormal blastocyst (‘39 mm. diam.) from the same batch, the cellular wall appears complete and normal, but the blastocyst cavity contains a group of about sixteen spherical cells averaging about ‘032 mm. in diameter, and in yet another abnormal egg ot' the same diameter and batch there is present an incomplete layer of flattened cells over one hemisphere, and towards the opposite pole of the eggsphere there occurs a group of spherical cells of variable size and some of them multinucleate. In this abnormal egg it appears as if the formative cells had divided in fairly normal fashion, whilst the nou-formative cells had failed to do so.
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Contents: 1 Review of Previous Observations | 2 The Ovum of Dasyurus | 3 Cleavage and Blastocyst | 4 Blastocyst Growth Ectoderm Entoderm | 5 Early Stages of Perameles and Macropus | 6 Summary and Conclusions | 7 Early Mammalia Ontogeny | Explanation of Plates
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