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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    common stock, itself of Prototlierian derivation), and bearing in mind the occurrence of an undoubted repi’esentative of the shell round the Marsupial ovum, I venture to see in the fluid- material of the deutoplasmic zone the partial and vestigial equivalent of the yolk-mass of the monotreme egg. In other words, I would regard the deutoplasmic fluid as the product of an abortive attempt at the formation of such a solid yolk- mass. The objection will no doubt be forthcoming that this interpretation cannot possibly be correct since the supposed equivalent of the yolk-mass in the Dasyure ovum is located, on my own showing, at the wrong pole—at the upper instead of at the lower. But its precise location does not seem to me to be a matter to which we need attach any great importance, since it has doubtless been adaptively determined in correla- tion with the special character of the cleavage process. The belief that the minute yolk-poor ovum of the Eutheria is no pure primarily holoblastic one, but that it has only secondarily arrived at the total type of cleavage as the result of the all but complete loss of the yolk ancestrally present in it, consequent on the substitution of the intra-uterine mode of development for the old oviparous habit, is now widely held amongst Mammalian embryologists. Hubi’echt, however, is an exception, wedded as he is to a belief in the direct deriva- tion of the Eutheria from Protetrapodous ancestors with yolk- poor, holoblastic eggs. Whether the interpretation I have put forward, viz. that the non-formative or deutoplasmic zone of the Dasyure ovum is the reduced and partial equivalent of the yolk-mass of the Monotreme egg, be accepted or not, I venture to think that my discovery of an actual elimination of deuto- plasmic material by the Marsupial ovum affords a striking confirmation of the truth of the prevailing conception as to the phylogeny of the Eutherian ovum, and I further venture to think that the facts I have brought forward in the preceding ])ages justify us in' regarding the ripe ovarian ovum of Dasyurus as being potentially of the yolk-laden, telolecithal type, and the uterine ovum, by bodily casting out the super- fluous part of its deutoplasm, as becoming at the same time