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Notes on the Development of the Germ-Layers in Diprotodont Marsupials

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Notes on the Development of the Germ-Layers in Diprotodont Marsupials Notes on the Development of the Germ-layers in Diprotodont Marsupials. By T. Kerr, M.A. Assistant in the Zoology Department, Queen's University of Belfast. With 5 Text-figures. THE material for this investigation was collected in Tasmania by Professor T. Thomson Plynn from animals taken in the wild during 1920 to 1929, and forms part of the marsupial and mono- treme material obtained by him by the aid of grants allowed by the Ealston Trustees and by the Grants Committee of the Royal Society. The author would like to express now his gratitude both for the opportunity of examining it and for the help and encouragement he has received during the course of the work. The material itself consists of nine series of sections of blasto- cysts of Bettongia cuniculus cut by Professor Flynn him- self in Tasmania, and of fourteen blastocysts of Bettongia cuniculus and four of Potorous tridactylus which have been cut here. A series of early stages of these rather scarce little Marsupials is extremely hard to obtain, for only a single egg is produced at a time and it is almost impossible to breed the animals in captivity (Plynn, 1930). The blastocysts were fixed in Bouin or in Hill's fluid (picro-nitric-aceto-osmic mix- ture), and those not cut at the time were preserved in alcohol; the whole series covers a range of development from an early unilaminar blastocyst to a fully formed primitive streak. The process of section cutting is apt to be exceptionally troublesome owing to the small size of the blastocysts and the thickness of the shell-membrane, but the following method proved very satisfactory. The blastocysts Avere impregnated with thin celloidin, fixed to small pieces of liver by means of the celloidin, and embedded in paraffin; the blocks were cut by the excellent large precision microtome of Bausch and Lomb, kindly presented to the Department by the Rockefeller Trustees. 306 T. KERB All sections were cut at 5/x, and stained with iron haematoxylin and eosin; as it was not possible to orientate the smaller blasto- cysts, doubtful points in the sections had to be cleared up by reconstructions. The development of the germ-layers in Diprotodont Marsu- pials has not been worked out, so it was decided to treat this part of the material separately from that dealing with the primi- tive streak. There are various accounts of the early develop- ment of Polyprotodonts. Selenka's (1887) work on Didelphys virginiana was based on a very limited amount of material, some of which was certainly abnormal. An abnormal eight- celled egg, consisting of two irregular rings of four cells each, misled him into considering the four somewhat smaller cells ectoderm and the larger endoderm. In his forty-two- and sixty- eight-celled blastocysts, or 'gastrulae' as he called this stage, there is a definite polarity, which he took to be the continuing differentiation of his smaller-celled 'ectoderm' and larger-celled 'endoderm'. Each of these blastocysts has included in if a large ' L'rentoderm' cell, and Selenka apparently believed that the endoderm was formed by the gradual inclusion into the cavity of all the cells of his ' endoderm' pole from the region of a 'blastopore', their places being taken by encroaching 'ecto- derm'. Possibly these 'Urentoderm' cells are similar to those described by Hartman (1919) as being included from about the fifty-celled stage onwards, and claimed by him also as endo- derm, but Selenka's general views are certainly incorrect. In the above paper Selenka goes on to describe some primitive streak stages from Didelphys, but he did not have any blastocysts showing the origin of the mesoderm. In 1892 he published a description of the development of Hypsipry- mnus cuniculus ( = Bettongia cuniculus), but in the earliest stage with which he deals, namely a blastoeyst two days old and 2 mm. in diameter, the primitive streak is already well formed. Hill (1910) gave a very full account of early develop- ment in some Polyprotodonts, describing in detail the formation of endoderm in Dasyurus viverrinus. In this animal smaller, darker-staining 'endoderm mother cells' develop here and there throughout the formative area of the unilaminar GERM-LAYERS IN DIPROTODONTS 307 blastocyst, and when the blastocyst is about 4-5 mm. in diameter these give rise to the true internal endoderm. .This may occur in two ways: in the first, the endoderm mother cell prolongs itself along the inner side of the embryonic ectoderm, then migrates bodily out of place and comes to lie on the inner surface of the ectoderm; in the second, the prolongation divides to form an endoderm cell before the mother cell migrates. In either case the cells are capable of amoeboid activity, by which their migration is probably achieved, and of developing pseudo- podia, which by secondary anastomoses with each other form the beginnings of a continuous, though at first fenestrated, endoderm layer. Eepeated cell division closes up the layer and extends it completely round the inside of the blastocyst. From shorter series of specimens Hill considers the method of endo- derm formation in Macropus and Perameles to be similar; it is interesting to note, however, that in these the endo- derm first appears in 0-85 mm. and just over 1 mm. diameter blastocysts respectively. Minot (1911) in a brief description of a late bilaminar blastocyst of the opossum added nothing new. Hartman (1916) studied the egg of Didelphys virginiana up to the fully formed bilaminar blastocyst; in this paper he describes endoderm appearing in blastocysts of 0-6 mm. or less, containing somewhat over 100 cells, and his conclusions regard- ing its origin are substantially the same as those of Hill. He mentions the occurrence in earlier stages of large cells included in the cavity of the blastocyst, as does Hill, but is inclined to agree with the latter that they are of no morphological impor- tance. In a subsequent paper (1919), however, for which he had available a much more complete series of stages, he modifies his conclusions very considerably. He believes, in fact, that the included cells are the first appearance of the endoderm, which therefore appears at the fifty- or sixty-celled stage (about 0-15 mm. in diameter). He describes this as occurring by the rounding off of certain cells in a part of the wall representing the future embryonic area, and by their consequent detachment from the wall and their division to form endoderm. Hence the phenomena of endoderm formation described by Hill as the complete process in Dasyurus only represent the last stage 308 T. KERR in Didelphys. Hill (1918) described various stages of Didelphys aurita, but none dealing with layer formation. The following is a list of the various stages dealt with in this paper. When the blastocysts were in their natural spherical condition the diameter is given, when they were at all out of shape maximum and minimum measurements are given. Bettongia stages: I, 0-180 mm.; II, 0-210 mm.; Ill, 0-218 mm.; IV, 0-225 mm.; V, 0-266 mm.; VI, 0-275 mm.; VII, 0-308 mm.; VIII, 0-364 mm.; IX, 0-370 mm.; X, 0-380 mm. ; TEXT-FIG. 1. ALB., albumen; COAO., coagulum; SH.M., shell-membrane; Y.B., yolk-body. X 200. XI, 0-392 mm.; XII, 0-704 by 0-432 mm.; XIII, 0-728 by 0-304 mm.; XIV, 0-761 by 0-680 mm.; XV, 0-938 mm.; XVI, 1-324 by 0-993 mm.; XVII, 1-600 mm. Potorous stages: I, 0-208 mm.; II, 0-458 by 0-396 mm.; III, 2-622 by 2-001 mm. The earliest blastocyst in the Bettongia series is a young unilaminar (0-180 mm.) in which the nuclei are large and ill defined, and both albumen coat and central mass are still well developed. The central mass can be differentiated into coagu- lum and yolk-body; but, as often happens, the zona pellucida has broken down under fixation (Text-fig. 1). The next three stages are simple unilaminar blastocysts, showing an increase in the number of nuclei with a decrease in their size; in all the albumen coat and central mass are distinguishable, though increasingly attenuated. It is not possible to make out any definite polarity in these stages, although it is shortly to appear. GERM-LAYERS IN DIPROTODONTS 309 The examination of unsectioned blastocysts is unfortunately not possible, owing to their small size in Bettongia and their thick shells; among other things this makes it difficult to be sure of the shape of the endoderm cells when they arise, and impossible to say whether they have pseudopodia or not. In stage V (0-266 mm.) the albumen has almost disappeared, TEXT-FIG. 2. ALB., albumen; FORM. AR., formative area; PREC, precipitate; Z.P., zona pelluoida. X 250. but the zona pellucida can be clearly seen. The formative area has made its appearance as a concentration of cells forming a cap at one pole, the cap covering rather less than a quarter of the inside of the shell membrane (Text-fig. 2). The two areas remain distinct throughout succeeding stages, an increasing difference in thickness developing, and the endoderm mother cells when they appear are always within the formative portion. In this stage, which contains about eighty cells, and in many later ones, are present the inclusions noticed in early blastocysts by Hill and Hartman, and deemed by the former to be of no morphological importance inDasyurus. It was of the greatest interest, therefore, to discover whether these inclusions, or some of 310 T.
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