20 J. P. HILL. 



common stock, itself of Prototherian derivation), and bearing 

 in mind the occurrence of an undoubted representative 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 eg-g. 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 3'olk 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. Hubrecht, 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 Eiitherian 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 



