Paper - Contributions to the embryology of the marsupialia 4-5
|Embryology - 28 Nov 2020 Expand to Translate|
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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.
|James Peter Hill describes marsupial development in the native cat (Dasyurus Viverrinus)
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Chapter V. - Some Early Stages op Perameles and Macropus
The early material of Perameles and Macropus at my disposal comprises only a small number of stages, but is of special importance, since it enables me to demonstrate that so far as these particular stages are concerned, the early developmental phenomena in these forms are essentially the same as in Dasyurus, and thus affords ground for the belief that there is one common type of early development throughout the series of the Marsupialia. Moreover, it is of interest since it reveals the. existence of what might be termed specific differences in the early development of these Marsupials, especially in regard to the time of appearance of the entoderm. In Dasyurus, it will be remembered, the primitive entoderm cells first become definitely recognisable as internally situated cells in vesicles 4â€˜5 mm. in diameter. In Perameles they occur in vesicles just over 1 rnm. in diameter, while in Macropus they are already present in a blastocyst only -35 mm. in diameter, so that it would appear that the entoderm is differentiated much earlier in the higher, more specialised types than in the more generalised forms. This difference in time of appearance of the entoderm is perhaps to be correlated with a difference in size of the ovarian ova in the three genera mentioned.
The earliest material of Perameles I possess consists of two eggs of P. obesula, which I owe to the skill and enthusiasm of my friend Mr. S. J. M. Moreau, of Sydney. Egg -Ameasures '23 mm. in diameter, and egg B, â€˜24 x â€˜23 mm. The former consists of thirty-two cells, the latter of thirty. In both the shell-membrane has partially collapsed, but the general plan of arrangement of the blastomeres can still fairly readily be made out. Fig. 51, PI. 3, represents a micro-photograph of a section of egg B, the better of the two. It shows the .shell-membrane (nearly '005 mm. thick) externally, considerable remains of the albumen between that and the deeply stained zona, and then, closely applied to the inner surface of the latter, the blastomeres arranged in the form of an inverted D, so as to enclose a central space, open below as the figure stands. This latter opening extends through the series, and it seems probable that there was a corx*esponding one opposite to it in the intact egg. Evidently we have have a stage in the formation of the blastocyst, in which the blastomeres are in course of spreading towards one or both of the poles of the sphere formed by the egg-envelopes just as happeus in the corresponding' stage of Dasyurus (cf. fig. 51 with fig. 31j though the latter represents a somewhat older stage in Dasyurus). The blastocyst-wall here appears relatively more extensive than in the 32-celled stage of Dasyurus, an apparent difference which may perhaps be accounted for by the difference in size of the respective eggs (â€¢24 mm. as compared with '36 mm.) . The blastomeres situated adjacent to the opening and those on the right side of the figure tend to be more flattened and of greater superficial extent than the remainder, but I can recognise no difference in the cytological characters of the cells. The space or cleavage cavity enclosed by the blastomeres is partly occupied by a granular coagulum, and towards the opening there is present a lightly staining reticular mass, which i*ecalls the yolk-body of Dasyurus, though I am not prepared to affirm that it is of that significance. The fixation of the specimen is not quite perfect.
My next stage of Perameles is constituted by a blastocyst of P. nasuta, for which I am again indebted to Mr. Moreau measuring in the preserved condition '29 x '26 mm. Pig. 52, PI. 3, shows a section of this blastocyst. Structurally, it corresponds in all essential respects with the '43 mm. blastocyst of Dasyurus, figured on the same plate (fig. 33). The blastocyst Avail is complete and unilamiuar throughout. It is distinguishable into tAvo regions, a more extensive region over Avhich the cells are large and flattened and a less extensive, composed of smaller but thicker cells (left side of fig. 52). In the early blastocysts of Dasyurus, it may be recalled, the evidence showed that the region of more flattened cells is formative in significance, that of more bulky cells, non-formative. It is possible the same holds good for this Perameles blastocyst. On the other hand, the structural condition of the stage next to be described rather supports the vieAv that the smaller region, composed of plumper cells, is in this case formative. That view seems to me the more probable of the two, but there is a considerable difference in size betAveen the present blastocyst and those next available, so that it is impossible to decide this point witli certciinty. The blastocyst cavity is partly occupied by coagulnm. There are no cells present in it, but the question of the presence of a yolkbody must remain open. The shell-membrane (â€˜0045 mm. in thickness) and zona are in close apposition.
Following this early blastocyst, I have three vesicles of P. nasuta, two of them measuring 1â€˜3 mm. in diameter, the other PI mm. In their stage of development they agree pretty closely with the 4'5-5 mm. vesicles of Dasyurus, referred to in the preceding pages under the designation 6, '04, the entoderm being in process of differentiation. The formative region was readily distinguishable in the intact vesicles as a darker patch occupying about three eighths of the surface extent of the wall. In section (PI. 8, figs. 80, 81) it is characterised by its greater thickness as compared with the non-formative or trophoblastic region, and by the presence below it of numbers of primitive entodernial cells. Compared with the corresponding stage in Dasyurus, the chief difference consists in the relatively much greater thickness of the cells of the formative region in the Perameles vesicle. The latter cells are here already more or less definitely cubical in shape, their thickness varying from '09 mm. to '015 mm., and altogether they form a layer of a much more uniformly thickened character than that of the 6, '04 vesicles of Dasyurus. The trophoblastic ectoderm (figs. 80, 81, tr. ect.) is composed of somewhat flattened cells, .varying in thickness from â€˜005 to '008 mm.
The primitive entodermal cells (figs. 80, 81, ent.) are present below the formative region in fair abundance, more especially around the periphery of the same, which may thus appear somewhat thickened (fig. 81). 4'he cells vary in size from â€˜01 X '007 mm. to '024 x '009 mm., and they stain on the whole somewhat more deeply than the formative cells, to whose under-surface they are closely applied. They occur groups. Mitotic figures are frequently met with in the cells of the formative ai'ea (observe the obliquely disposed figure in one of the formative cells in fig. 81), and they also occur in the primitive entodermal cells. Examination of the sections leaves no doubt in one's mind as to the source of the entodermal cells. They are undoubtedly derived from the formative region of the vesicle wall. The 'shellmembrane has a thickness of about '0027 mm.
Of Macropus the earliest stage I have examined is a blastocyst of M. ruficollis, -25 x *21 mm. in diameter. It is not in a quite perfect state of preservation, but is in a sufficiently good condition to enable me to say that the wall is complete and unilaminar throughout, just as in the â€˜29 x "26 mm. blastocyst of Perameles. The shell-membrane has a thickness of about -005 mm., and there are still remains of the albumen between it and the zona.
My next stage (figs. 82-85) is a blastocyst of the same species, *35 mm. in diameter. It unfortunately suffered in preparation, but practically the whole of the formative area of the blastocyst wall and part of the trophoblastic ectoderm are comprised in the sections (PI. 9, fig. 82), so that it is still possible to make out its chief structural features. In its stage of development this blastocyst closely agrees with the last described blastocysts of Perameles. The formative area of the wall is perfectly distinct in the sections because of its greater thickness and the presence below it of the primitive entodermal cells. It attains its gi-eatest thickness (*027 mm.) peripherally, whilst it is thinnest centrally (*006 mm.), so that, taken as a whole, it is not quite such a uniformly thickened layer as is that of the Perameles blastocysts. Primitive entodermal cells are present below it, but not in great abundance (figs. 82, 84, 85, ent.). In fig. 83, a formative cell is seen in division, the axis of the spindle being oblique to the surface. The trophoblastic ectoderm (figs. 82, 83, tr. ect.) is composed of the usual flattened cells, and varies in thickness from 005 to 0067 mm.
In the blastocyst cavity, adjacent to the trophoblastic ectoderm on the left side of fig. 82, there is visible a small spherical cell similar to the degenerate cells met with in blastocysts of Dasyurus.
My last stage of M. ruficollis comprises an excellently preserved blastocyst, measuring '8 mm. in diameter, in which the embryonal ectoderm and the entoderm are definitely established. It thus corresponds to the 8, '01 stage of Dasyurus (blastocysts o - 5'5 mm. diameter). The embryonal area is circular and measures '468 mm. in diameter. Its constituent cells are cubical and from '008 to â€˜OlS mm. in thickness, Avhilst the trophoblastic ectoderm is formed of flattened cells, -006 ram. in thickness. The entoderm is present as a continuous layer of attenuated cells below the embryonal ectoderm, and it probably also forms a continuous layer below the trophoblastic ectoderm. Entodermal cells are certainly pi*esent over the lower polar region of the vesicle, but it is difficult to be certain from the sections whether or not they form a perfectly continuous layer. The shell membrane has a thickness of â€¢0026 mm.
I have a corresponding blastocyst of Petrogale penicillata â€¢915 mm. in diameter, with an oval, embryonal area â€¢525 X ^45 mm. in diameter, and a later blastocyst of M. ruficollis P46 mm. in diameter, with a circular embryonal area '57 mm. in diameter.
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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
Cite this page: Hill, M.A. (2020, November 28) Embryology Paper - Contributions to the embryology of the marsupialia 4-5. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Contributions_to_the_embryology_of_the_marsupialia_4-5
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