104 J. 1>. HILL. 



homology need no longer be doubted. In the preceding section 

 of this paper (ante, pp. 91, 92) I have shown reason for tlie 

 conclusion that the non-formative region of the Marsupial 

 bUistocyst is the homologue of tlie extra-embi'vonal ectoderm 

 of the jMonotreme and Reptile, and if that conclusion be 

 accepted it follows that tlie outer enveloping layer of the 

 Eutherian blastocj'st, the so-called trophoblast of Hubrecht, 

 is none other than extra-embryonal ectoderm, as maintained 

 by Van Beneden, Keibel, Bonnet, Jenkinson, Lee, MacBride 

 and others, the homologue of that of Reptilia. 



I am therefore wholly unable to accept the highly specula- 

 tive conclusions of Hubrecht, set forth with such brilliancy 

 in a comparatively recent number of this Journal ('08), as 

 to the significance and phylogeny of this layer. These con- 

 clusions, on the basis of which he has proceeded to formulate 

 such far-reaching and, indeed, revolutionary ideas not only 

 on questions enibrj^ological, but on those pertaining to the 

 phylogeny and classification of vertebrates, have already 

 been critically considered by Assheton ('09) and MacBride 

 ('09), also in the pages of this Journal, and found wanting, 

 and they are, to my mind, quite irreconcilable with the facts 

 I have brought to light in regard to the early development 

 of Marsupials. I yield to no one in my admiration for the 

 epoch-making work of Hubrecht on the early ontogeny and 

 placentation of the Mammalia, and I heartily associate 

 myself with the eulogium thereanent so admirably expressed 

 by Assheton in the critique just referred to (p. 274), but 

 I am bound to confess that as concerns his views on the 

 phylogeny of this layer, which he has termed the "tropho- 

 blast," he seems to me to have forsaken the fertile field of 

 legitimate hypothesis for the barren waste of unprofitable 

 speculation, and to have erected therein an imposing edifice on 

 the very slenderest of foundations. 



J^efore I proceed to justify this, my estimate of Hubrecht's 

 views on the phylogeny of the trophoblast, let me first set 

 forth his conception so far as I understand it. He starts 

 with the assumption that the vertebrates (with the exception 



