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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    animal pole of the egg, the opposite pole in relation to which the formative cytoplasm is situated being' the lower or vegetative. The deutoplasmic cytoplasm thus lies in the upper hemisphere, whilst the formative cytoplasm occupies the lower. If Van der Stricht’s determination of the poles of the ovum of Vesperugo be accepted, then we must conclude that the poles of the Dasyurus ovum are exactly reversed as compared with those of the bat^s egg. In this connection it may be recalled that Lams and Doorme [’07] have demon- strated the occurrence in Cavia of an actual reversal of the original polarity of the ovum, pi’ior to the beginning of cleavage. These facts may well give us pause before we proceed to attach other than a purely secondary significance to the exact location of the formative and deutoplasmic con- stituents in the Metatherian and Eutherian ovum. But besides this apparent difference in the location of the deuto- plasmic constituents of the ova of Dasyurus and Vesperugo, there exists yet another which concerns the fate of these con- stituents in the respective eggs. In Vesperugo, Van der Stricht shows that the “ deutoplasm ” remains an integral part of the egg, and I’etains its polar distribution in the blastomeres up to at least the 4-celled stage.^ In Dasyurus, on the other hand, the fate of the deutoplasmic mass is a very different, and, indeed, a very remarkable one. It does not remain an integral part of the segmenting egg as in Vesperugo, but prior to the completion of the first cleavage furrow it becomes bodily separated off, apparently by a process of abstriction, from the formative cytoplasm as a clear rounded mass which takes no further direct part in the developmental processes. As soon as its elimination is effected, the remainder of the cytoplasmic body of the ovum, formed of the formative cytoplasm alone, divides into the first two equal-sized blasto- meres, the first cleavage plaue being coincident with the polar diameter and at right angles to the plane of separation of the deutoplasmic mass, or “yolk-body ” as we may term it (PI. 2, figs. 14-16, 19, ij.b.), so that it is this formative zone of the * Vide, however, “Addendum” (p. 121).