102 J. r. Hiix. 



differs from all other observers iu holding that the outer 

 enveloping layer of the same is entodermal.^ 



The fact, then, of the occurrence amongst Eutheria of a 

 "precocious segregation " of the blastomeres into two distinct 

 groups, one of which eventually surrounds the other com- 

 pletely, is not in dispute, though authorities differ widely in 

 the interpretation they place upon it. In the Eutherian 

 blastocyst stage, the enveloping layer forms the outer uni- 

 laminar wall of the vesicle, and encloses the blastocyst cavity 

 as well as the other internally situated group. This latter 

 typically appears as a rounded cell-mass, attached at one spot 

 to the inner surface of the enveloping layer, but more or less 

 distinctly marked off from it. It is generally termed the 

 inner cell-mass or embryontil knot ("amas cellulaire interne" 

 of Van Beneden). For the envelopitig hiyer Ilubrecht's name 

 of " trophoblast " is now generally employed, even by those 

 who refuse to adopt the speculative views with which its 

 originator has most unfortunately, as I think, enshrouded this 

 convenient term. 



I have demonstrated the occurrence of an apparently com- 

 parable "precocious segregation" of the blastomeres into 

 two distinct groups in one member of the Metatheria which 

 there is no reason to regard as an aberrant type, and I have 

 shown beyond all shadow of doubt that from the one group, 

 which constitutes what I have termed the formative region 

 of the unilaminar vesicle-wall, there arise the embryonal 

 ectoderm and the entire entoderm of the vesicle, both em- 

 bryonal and exti-a-embryonal, and that the other group, which 

 constitutes the non-formative region of the vesicle-wall, 

 directly furnishes the extra-embryonal ectoderm, i.e. the 

 ectoderm of the omphalopleure and chorion.- 



' Assheton states ("08. p. 233, cf. also '98, p. 220) that his interpreta- 

 tion " owes miicli also to the theoretical conchisions of Minot and 

 Robinson." However that may he, hoth Minot and Roljinson in their 

 most recent writings continue to speak of the chorionic ectoderm. 



'■' "Whether or not it participates in the formation of the amniotic 

 ectoderm futm-e investigation nuist decide. 



