THE EARLY DEYELOPMEXT OF THE INFAESUPIALIA. 97 



bat a slightly modifieil version of the Prototherian, such 

 differences as exist between them being- intei'pretable as coeno- 

 geuetic modifications, induced in the Metatherian by the 

 practically complete substitution of uterine nutriment for 

 intra-ovular yolk, a substitution which has resulted in the 

 attainment by the marsupial ovum of the holoblastic tj'pe of 

 cleavage. In tlie present section I hope to demonstrate how 

 the early ontogeny of the Metatlieria enables us to interpret 

 that of the Eutheria in terms of that of the Prototheria. 



If Ave proceed to compare the early development in the 

 Metatheria and Eutheria, we encounter, from the 4-celled 

 stage onwards, such obvious and profound differences in the 

 mode of formation of the blastocyst, and in the relations of 

 its constituent parts, that the differences seem at first sight 

 to far outweigh the resemblances. Nevertheless, apart from 

 their common possession of the same holoblastic mode of 

 cleavage, there exists one most striking and fundamental 

 agreement between the two in the fact that in both there 

 occurs, sooner or later during the cleavage process, a separa- 

 tion of the blastomeres into two distinct, pre-determined cell- 

 groups, whose individual destinies are very different, but 

 apparently identical in the two subclasses. In tiie Marsupial, 

 as typified by Dasyurus, the fourth cleavages are, as we have 

 seen, unequal and qualitative, and result in the separation of 

 two differentiated groups of blastomeres, an-anged in two 

 superimposed rings, viz. an upper ring- of eight smaller, less 

 yolk-rich cells, and a lower of eight largei", more yolk-i'ich 

 cells. The evidence justifies the conclusion that the former 

 gives origin directly to the formative or embryonal region of 

 the vesicle wall, the latter to the non-formative or extra- 

 embryonal region. 



Amongst the Eutheria the evidence is no less clear. It has 

 been conclusively shown by various observers (Yan Beneden, 

 Duval, Assheton, Hubrecht, Heape, and others) that, sooner 

 or later, there occurs a separation of the blastomeres into two 

 distinct groups, one of which eventually encloses the other 

 completely. The two groups may be clearly distinguishable 



VOL. 56, PART 1. NEW SERIES. 7 



