THE EAELY DEVELOPMENT OF THE MARSUPIALIA. 103 



As regards Eutheria, we have seen that Van Beneden and 

 Hubrecht, though their views in other respects are widely 

 divergent^ both agree that the inner cell-mass of the blasto- 

 cyst furnishes the embryonal ectoderm (as well as the amniotic 

 ectoderm wholly or in part) and the entire entoderm of the 

 vesicle. That, in fact, is the view of Mammalian embryologists 

 generally (Duval and Assheton excepted)/ and if Ave may 

 assume it to be correct, then it would appear that the later 

 history of the formative region of the Marsupial blastocyst 

 and that of the inner cell-mass of the Eutherian are identical. 

 That being- so, and bearing in mind that both have been 

 shown, at all events in certain Mammals, to have an identical 

 origin as a group of precociously segregated blastomeres,- I 

 can come to no other conclusion than that they are homo- 

 genous formations. If that be accepted, then this fact by itself 

 renders highly probable the view that the so-called tropho- 

 blast of the Eutherian blastocyst is homogenous with the 

 non-formative region of the Metatherian vesicle, and when 

 we reflect that both have precisely the same structural and 

 topographical (not to mention functional) relations in later 

 stages, inasmuch as they constitute the ectoderm of the chorion 

 and omphalopleure (with or without participati(-»n in the 

 formation of the amniotic ectoderm j, and that both have a 

 similar origin in those ^lammals in which a precocious segre- 

 gation of the blastomeres has been recognised, their exact 



' The view of Duval ['95]. based on the study of Yespertiho. that the 

 inner cell-mass gives rise solely to entoderm, and that the enveloping 

 layer furnishes not only the extra-embryonal hut also the embryonal 

 ectoderm, is sho'5\T.i by Van Beneden's observations on the same form to 

 be devoid of any basis of fact. Assheton's views are referred to below 

 (p. IIU). 



- The fact that the phenomenon of the " precocious segregation " of 

 the blastomeres into two groups with determinate destinies has already 

 become fixed in the Marsupial lends additional weight to the view of 

 Van Beneden that such a segregation will eventually be recognised as 

 occurring in all Eutheria without exception. Without it, it is difficult 

 to understand how the entypic condition, characteristic of the blasto- 

 cysts of all known Eutheria, is attained, imless by differentiation in 

 situ, which seems to me highly improbable. 



