THE EAKLY DEVELOrMENT OF THE WAKSUPIALIA. 107 



('08, p. 151), and ^'for Cyclostomes the same reasoning liolds 

 good" (p. 152). 



The trophobhist, then, is conceived of by Hubrecht as a 

 lai-val nieinbrane of ectodermal derivation, which invests the 

 embryonal anlage in all Vertebrates with the exceptions 

 mentioned, which is snbject to secondary reduction, and which 

 is homologous throughout the series. As I understand the 

 conception, Avhat is ordinarily called extra-embryonal ecto- 

 derm in the Sauropsida is not trophoblast, otherwise Hubrecht 

 could hardly write — " in reptiles and birds traces of the 

 larval layer have in late years been unmistakably noticed" 

 ('09, p. 5) ; nevertheless what other writers have termed 

 embryonal and extra-embryonal ectoderm in the Prototheria 

 is claimed by Hubrecht as trophoblast (at all events that is 

 my interpretation of his statement that a trophoblastic vesicle 

 is present in these forms), and yet some years ago Hubrecht 

 ('04, p. 10) found it difficult " to understand that the name 

 has been misunderstood both by embryologists and gynasco- 

 logists." My own feeling is that the more recent develop- 

 ments in his views have tended to obscure rather than to 

 clarify our ideas as to the trophoblast, especially if we must 

 now hold that the chorion or serosa of the Saurqpsida is not 

 homologous with that of the Prototheria, which necessarily 

 follows if the extra-einbryonal ectoderm of the Sauropsidan is 

 not the same thing as that of the Monotreme. 



Assuming that we have formed a correct conception of the 

 trophoblast as a larval membrane, and bearing in mind that it 

 is best developed in the Metatheria and Eutheria, since these 

 alone amongst higher Vertebrates have retained unaltered 

 the viviparous habits of their Protetrapodous ancestors, let us 

 see what basis in fact there is for the statement of Hubrecht 

 ('08, p. GS) that "before the ectoderm and the entoderm 

 have become differentiated from each other there is in 

 mammals a distinct larval cell-layer surrounding (as soon as 

 cleavage of the egg has attained the morula stage) the 

 mother-cells of the embryonic tissues." Now that statement 

 as it stands, I have no hesitation in characterising as entirely 



