Paper - Contributions to the embryology of the marsupialia 4-1
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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 I. - Critical Eeview of Previous Observations on the Early Development of the Marsupialia
Apart from the very brief abstract of a short paper on the development of Dasyurus, which I read before Section D of the British Association in 1908 (included in Dr. Ashworth's Report, ^Natui'e,' vol. Ixxviii), our knowledge of the processes of cleavage and germ-layer formation in the Marsupialia is based (1) on the well-known observations of the late Emil Selenka ('86) on the development of the Virginian opossum (Didelphys mars upialis), published in 1886 as Heft 4 of his classical 'Studien'; and (2) on those of W. H. Caldwell ('87) on the uterine ovum, and cleavage process in the native bear (Phascolarct us cinereus).
Selenka's account of the mode of origin of the germ-layers in Didelphys differs widely, us the sequel will show, from my description of the same in Dasyurus. Now Didelphys and Dasyurus are two marsupials, admittedly allied by the closest structural ties, and we should therefore not expect a priori that they Avould differ fundamentally in the details of their eai'ly ontogeny, however much they might diverge in respect of the details of their embryonal nutritional arrangements.
Furthermore, we might reasonably hope, in view of the generally admitted relationships of the Marsupialia, that a knowledge of their early development would aid us in the interpretation of that of Eutheria, or, at least, that their early developmental phenomena would be readily comparable with those of Eutheria. It cannot be said that Seleuka's observations realise either of these expectations. Whichever view is taken of Seleuka's description of the opossum,†writes Assheton ('98, p. 254), “ many obvious difficulties remain for the solution of which no satisfactory suggestion can as yet be offered.
As concerns niy own observations, I venture to think it is possible to bring* them into line with what we know of the early ontogeny in the other two mammalian sub-classes, and I have attempted to do so in the concluding chapter of this paper, with what success the«‘eader can judge, whilst as regai'ds the divergence between Selenka's results and my own, I am perfectly convinced that the explanation thereof is to be found in the fact that the whole of Selenka's early material was derived from but two pregnant females, and that much of it consequently consisted of eggs which had failed to develop normallj". From the one female, killed 5 days after coition, he obtained one egg in the 2-celled stage, one with about twenty cells and nine unfertilised ova. From the second, killed 5 days 8 hours after coition, he obtained “ ausser zwei tauben, 14 befruchtete Eier niimlich je ein Ei mit 4, 8, 42, 68 Zellen, eine junge und eine iiltere Gastrnla mit noch dicker Eiweisschicht und endlich acht auch gleichen Entwickelungsstufe stehende Aveit grossere Keimblasen, deren AVand noch grosstentheils einschichtig war†('86, p. 112). Selenka recognised that the last-mentioned blastocyst “die normale Entwickelungsphase reprasentiren,†since he found as a rule that all the embryos from one uterus Avere in the same developmental stage. Nevertheless he proceeded to describe the segmenting eggs and the two “ gastrulas †Avhich lagged so far behind the blastocysts, as if they Avere perfectly normal developmental stages. He does, indeed, question Avhether or not the 42-celled stage is normal, but decides in the affirmative, “ denn Avenn ich von zwei Zweifelhaften Fallen absehe, so habe ich niemals Eier aus den ersten Tag aufgefunden, Avelche auf irgend Avelche Anomalie der EutAvickeluiig hiiiAviesen.†This, hoAvever, can hardly be accepted as a satisfactory reason for his conclusion, since apart from the other eggs of the same batch, he had but the two eggs from the other female for comparison, viz. the 2-celled egg (and even that is, in my vieAV, not quite normal), and the 20-celled egg, Avhich is stated to have suffered in preparation. AVith the exception of the tAvo eggs just mentioned, all the crucial early stages (ranging from the 4-celled stage to the completed blastocyst), on whose examination Selenka based his account of germ-layer formation in Didelphys, would thus appear to have been derived from a single female.^ No wonder it is impossible to reconcile his description either with what we know of germ-layer formation in the Prototheria and Eutheria or with my account of the same in Dasyurus.
My own experience with the latter has shown me that no reliance whatever is to be placed on segmenting eggs or blastocysts which exhibit marked retardation in tbeir stage of development as compared with others from the same uterus, and also that batches of eggs or blastocysts in which there is marked variation in the stage of development attained should likewise be rejected. Abnormalities in the process of cleavage and of blastocyst formation are by no means uncommon in Dasyurus, and during the earlier stages of my own work I spent much time and labour on the investigation of just such abnormal material as that on which Selenka, no doubt unwittingly, but I feel bound to add, with an utter disregard for caution, based his account of the early development of Didelphys.
I propose now, before passing to my own observations, to give a short critical account of Selenka's observations, my comments being enclosed in square brackets.
The uterine ovum of Didelphys is enclosed by (1) a relatively thin “ granulosamembran,†formed by the transformation of a layer of follicular cells [really the shell-membrane, first coiTectly interpreted by Caldwell C87) and formed in the Fallopian tube] ; (2) a laminated hiyer of albumen, semitransparent ; (3) a zona radiata, not always recognisable [in my experience invariably distinct].
Cleavage begins in the uterus, is holoblastic, and at first equal. A 2-celled stage is figured (Taf. xvii, fig. 3) [not quite normal as regards the relations of the blastomeres], and also a 4-celled stage [normal in appearance except for the enormous thickness of the albumen layer], iu which the four equal-sized blastomeres are radially arranged round a cleavage cavity and are conical in form, their upper ends being more pointed, their lower ends thicker and richer in yolk-material. The nucleus of each is excentric, being situated nearer the upper pole. [This description is applicable word for word to the4-celled stage of Dasyurus.]
- The collection of my own early material of Dasyurus has involved the slaughter of over seven dozen females.
An 8-celled stage (fig. 6) is next described, seven of the blastomeres being equal in size and one being smaller. They are arranged somewhat irregularly iu two circles. [This stage I regard as abnormal both in respect of the arrangement of the blastomeres and the occurrence of irregularity amongst them.] Selenka (p. 119) thought it probable that the third cleavage planes cut the first two at right angles and divided each of the first four blastomeres into a smaller ectodermal cell and a larger more granular entodermal, but states that he was unable to establish this owing to the opacity of the albumen-layer. [My observations show that it is the fourth cleavage in Dasyurus, not the third, which is equatorial, unequal, and qualitative, and that even then the cells formed are not ectodermal and entodermal insignificance. The albumen is normally never opaque.]
A 20-celled stage is mentioned, but not described, since it suffered in preparation. It is said to have a large entoderm cell in the cleavage cavity. [A statement of very doubtful value, since the blastomeres were admittedly pressed together and probably displaced by the shrunken egg-membranes.]
The next stage described is a spherical “gastrula^' (Taf. xvii, figs. 7, 8), composed of fox'ty-two cells with an open "blastopore^' at the vegetative pole, a smaller opening at the animal pole, and a large “ ur-entoderm †cell in the cleavage-cavity, just inside the "blastopore.†The wall of the "gastrula†consists of cells graduated in size ; those in the region of the blastopore are the largest and richest in deutoplasm, those at the opposite pole are the smallest and most transparent. [This is a very characteristic stage in the formation of the blastocyst, with which I am quite familar in Dasyurus. Selenka's specimen, judging from Dasyurus, is normal as regards the constitution of its wall and the occurrence of an opening at each pole. The lower opening, however, has no blastoporic significance, but, like the upper, owes its presence to the mode of formation of the blastocyst-wall by the spreading of the blastomeres towards the poles of the sphere formed by the egg-envelopes. Selenka's blastopore simply marks the last point of closure. This specimen I hold to be abnormal from the presence of the so-called “ urentoderm †cell in its interior.- I figure (PI. 3, fig. 37) a section of a fairly comparable and undoubtedly abnormal blastocyst of Dasyurus in which there is also present in the blastocyst cavity a large free cell. Here this latter is unquestionably a blastomere of the lower hemisphere, which, having failed to divide, has become enclosed by the spreading of its neighbours. Selenka^s “ urentodermzelle I regard as a similarly displaced blastomere.]
A 68-celled “gastrula^' (figs. 9 and 10) is next described. It is essentially similar to the preceding, only the “blastopore †has closed.
The succeeding stage (fig. 11) is a somewhat older “ gastrula,†in which gastrulation is said to be still in progress, since over the lower pole, in the ]-egion of the now closed blastopore, it is no longer possible to say which cells belong to the ectoderm, which to the entoderm. The latter layer is described as being several cells thick in the blastoporic region, and as in course of spreading round inside the ectodermal wall of the“gastrula†to war ds the u pper o r ani mal pole. [This specimen is undoubtedly abnormal, at all events there is no comparable stage in Dasyurus. It is difficult to obtain a clear idea of Seleuka's conception of the mode of origin of the germ-layers, but he evidently held (cf . pp. 116 and 119) that the large yolk-rich cells of the lower (“ blastoporic â€) pole constitute the anlage of the entoderm, and that they become inturned at the “ blastopore †and pi'oliferate to form the definitive entoderm, which then gradually extends round to the animal pole, in contact with the inner surface of the wall of the gastrula., that wall forming the ectoderm. He apparently did not regard the “ urentodermzelle †as the sole progenitor of the entoderm, but simply as an entoderm-cell precociously inturned from the blastoporic †margin.
This view of Selenka, however, lands us in the predicament of having to regal'd the embryonal area as differentiating over the vegetative hemisphere, since in the next stage the ‘'blastopore^' is described as being situated excentrically in that ai'ea. Either Selenka's detei'mination of the poles in the 42-celled blastocyst is wrong, or the entoderm does not originate as he describes it. My own observations force me to accept the latter alternative. In his paper Selenka gets over the difficulty very easily by altering the orientation of his figures. On Taf. xvii, the figures of sections of blastocysts are so placed that the “blastopore †is below, next the bottom of the plate. These figures I hold to be correctly orientated. On Taf. xviii, the figui'es are inverted, so that the “blastopore†is above; as the result the animal pole of fig. 11, Taf. xvii, becomes the' vegetative pole of the stage next described (fig. 2, Taf. xviii).]
The stage just referred to, described as an “eiformige gastrula,†is represented in a drawing made from the fresh specimen as lying quite free in a large perivitelline space enclosed by a very thick layer of albumen, outside which is the “ granulosa-membran.†In section (fig. 2) a mass of entoderm is seen to reach the sm-face at one pole (marked hi.) uppermost in the figure, whilst other entodermal cells are shown spreading from this towards the lower pole. The ectoderm of the wall is represented as composed of definitely cubical cells. [The presence of a large perivitelline space, by itself stamps this specimen as not normal. The sectional figure must be schematic.]
The last of Selenka's early stages to which reference need be made here is formed by eight “ gastrulas†(blastocysts), reckoned as ten hours after the commencement of cleavage [a reckoning I consider of no value] (Taf. xviii, figs. 3 and 4). The embryonal area is now distinguishable by the larger size of its ectodermal cells. The entoderm is unilamiuar, and has extended beyond the limits of the embryonal area. The position of the blastopore is said to be marked in all by a mass of coagulum attached to the wall, and in three by a definite opening' as well. It is situated excentrically in the embryonal area. [Except for the ‘‘ blastopore and the presence of a thick layer of albumen, this blastocyst stage is quite comparable with the corresponding one in Dasyurus; the latter, however, is considerably larger. Of Selenka's early material, I think it is these blastocysts alone which had any chance of giving origin to normal embryos.]
W. H. Caldwell, who, as Balfour student, visited Australia in 1883-4, obtained a very rich collection of early marsupial Tnaterial, of which, unfortunately, no adequate account has ever been published. He gave, however, in his introductory paper on the ‘ Embryology of the Monotremata and Marsupialia^ (^87), an account of the structure of the ovum, both ovarian and uterine, in Phascolarctus, and he showed that the ovum during its passage down the Fallopian tube becomes enclosed outside the albumen layer in “a. thin transparent membrane, '0015 mm. thick,†which he homologised with the shell-membrane of the monotreme egg. This impoi*tant discovery of the existence of a shell-membrane in the Marsupialia I can fully confirm. I am, however, unable to accept his interpretation of the internal structure of the ovum of Phascolarctus, or his remarkable statement that cleavage in that form is of the meroblastic type. Cleavage is not described in detail, nor is any account given of the mode of origin of the germ-layers.
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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. (2024, April 24) Embryology Paper - Contributions to the embryology of the marsupialia 4-1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Contributions_to_the_embryology_of_the_marsupialia_4-1
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