THE EARLY DEVELOPMEXT OF THE MAESUPIALIA. 85 



embryonal kuofc of the Eutheriaii blastocyst ; and (2) that the 

 extra-embryonal or non-formative region of the same is the 

 homologue of the extra-embryonal ectoderm of the feaurop- 

 sida and Monotremata and of the trophoblast of the 

 Eutheria. 



As regards conclusion (1) there is not likely to be much 

 difference of opinion, but as regards (2), whilst perhaps the 

 majority of embryologists support the obvious, not to say 

 common-place view which I here advocate, it seems certain 

 tliat it Avill prove neither obvious nor acceptable to those 

 mammalian embryologists (I refer specifically to my friends 

 Professor A. A. W. Hubrecht and Mr. K. Asshetou) who, with 

 only Selenka^s account of early Marsupial ontogeny before 

 them, have formulated other and quite divergent views as to 

 the morphological nature of the outer enveloping layer of the 

 Eutherian blastocyst. It is therefore necessary to discuss 

 this question further, though I would fain express my convic- 

 tion that had the observations recorded in this paper been 

 earlier available, much vain speculation as to the phylogeuy 

 of the trophoblast might possibly have been avoided. 



Chapter VIL — The Eaely Ontogeny op the Mammalia in 

 THE Light of the Foregoing Observations. 



In entering on a discussion of the bearings of the results 

 of my study of the early development of ^larsupials on 

 current interpretations of early Mammalian ontogeny, and 

 especially of the homologies of the germ-layers, I desire at 

 the outset to emphasise my conviction that, specialised 

 though the Marsupials undoubtedly are in certain features of 

 their anatomy, e. g. their dentition, genital ducts, and mam- 

 mary apparatus, the observations recorded in the preceding 

 pages of this paper aii'ord not the slightest ground for the 

 supposition that their early ontogeny is also of an aberrant 

 type, devoid of signiticance from the point of view of that of 

 other mammals. On the contrary, I hope to demonstrate 

 that the Marsupial ty])e of early development not only readily 



