THE EAELY DEYELOPMEXT OF THE MARSDPIALIA. 95 



only of them, but, as Assheton has pointed out ("98, p. 251), 

 of the higher mammals as well (cf. the process of epiboly and 

 the inertness at first displayed by the formative cells of 

 the embryonal knot as compared with the activity of the non- 

 formative or tropho-ectodermal cells), an activity which 

 results in the rapid completion of that characteristically 

 mammalian developmental stage — the blastocyst or blasto- 

 dermic vesicle. 



The necessity for the early formation of such a stage, 

 capable of rapidly growing in a nutrient fluid medium 

 provided by the mother, has profoundly influenced the early 

 ontogeny in all three mammalian subclasses, and naturally 

 most of all that of the Eutheria, in which reduction of the 

 ovum, both as regards size and secondary envelopes, has 

 reached the maximum. And I think there can be little 

 doubt but that it is this necessity which has induced that 

 early separation of the blastomeres into two categories, 

 respectively formative and non-formative in significance, 

 which has long been recognised as occurring in Eutheria, and 

 which I have shown also occurs amongst the Metatheria. 

 This early separation of the blastomeres into two distinct 

 groups is not recognisable in the Sauropsida, and the idea 

 that it is in some way connected with the loss of yolk which 

 the mammalian ovum lias suffered in the course of phjdogeny, 

 Avas first put forward, I believe, by Jenkinson. In his paper 

 on the germinal layers of Vertebrata ('06, p. 51) he writes : 

 *' Segmentation therefore is followed in the Placentalia by 

 the separation of the elements of the trophoblast from those 

 destined to give rise to the embryo and the remainder of its 

 foetal membranes, and this 'precocious segregation' 

 seems to have occurred p h y 1 o g e n e t i c a 1 1 y during 

 the gradual loss of yolk which the egg of these 

 mammals has undergone." Whether or not such a 

 *•' precocious segregation " has already become fixed in the 

 Monotremes, future investigation must decide (cf. ante, p. 90). 



The loss of yolk, with resulting reduction in size which the 

 Monotreme ovum has suffered in the course of phylogeny, we 



