The early development of the marsupialia, with special reference to the native cat (dasyurus viverrinus) / by J.P. Hill.

  • Hill, J. P.
Date:
[1910?]
Catalogue details

Licence: In copyright

Credit: The early development of the marsupialia, with special reference to the native cat (dasyurus viverrinus) / by J.P. Hill. Source: Wellcome Collection.

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    men, judging from Dasyurus, is normal as regards the consti- tution 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 un- doubtedly 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 hemi- sphere, which, having failed to divide, has become enclosed by the spreading of its neighbours. Selenka^s “ urentoderm- zelle’'’ 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 “blasto- pore ” 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 ecto- dermal 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 ”) 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 appa-