110 J. I\ HILL. 



Hubrecht of the presence of a larva! membrane of the nature 

 postulated in the Prototlieria and Metatheria is devoid of 

 foundation in fact, so that there but remaius the question of 

 the significance of the outer enveloping layer of the Eutherian 

 blastocyst. As regards that, I venture to think that the 

 alternative interpretation of E. van Beneden and other 

 investigators, which I have attempted to develop in the 

 pages of tins paper, affords a simpler and more satisfying 

 explanation of its significance and phylogeny than that 

 advocated by Prof. Hubrecht, an interpi-etation, moreover, 

 which is more in accordance, not only with all the known 

 facts, but ''with sound evolutionary principles " and with the 

 conclusions arrived at by the gi'eat majority of comparative 

 anatomists and paleontologists as to the origin and inter- 

 relationships of the Mammalia. 



And I also venture to think tliat what has just been said 

 holds true with reference to the views advocated by Mr. 

 Assheton. These views owed tlieir origin to certain appear- 

 ances which he found in some segmenting ova of the slieep 

 (but, be it noted, not in all those he examiued), and he has 

 attempted to re-interpret not only his own earlier observations, 

 but those of other workers on the early ontogeny of the Eutheria 

 in the light of liis newer faith, and not only so, he holds that it 

 is also possible to apply that in the interpretation of the early 

 ontogeny of Marsupials (v. '08, p. 235, and '09, p. 229). He 

 maintains that the inner cell-mass of Eutlieria is purely ecto- 

 dermal, and that the enveloping trophoblast layer of the blasto- 

 cyst arises in common with the entodermal lining of the same 

 and is therefore also entodermal. " On tlie theory I advocate," 

 he writes ('09, p. 235), " the trophoblast is of Eutherian 

 mammalian origin only and is not homologous to any form of 

 envelope outside the group of Eutherian mammals." These 

 views of Assheton are not only nt variance with those of all 

 other investigators who have worked at the earl}^ ontogeny of 

 Eutheria, but they are quite irreconcilable with my observa- 

 tions on the development of Dasyurus herein recorded. I claim 

 to have shown in that Marsupial that the formative region, the 



