114 J. r. HILL, 



In the Mouotremes this vesiculai' stage is rapidly and 

 directly attained as the result, firstly, of the rearrangement 

 of the blastomeres of the cleavage-disc to form a unilaminar 

 blastodermic membrane overlying the solid yolk, and, secondly, 

 of the rapid extension of the peripheral (extra-embryonal) 

 resrion of the same, in contact with the inner surface of the 

 firm sphere furnished by the egg-envelopes. During the 

 completion of the blastocyst embryonal differentiation remains 

 in abeyance, and practically does not start until after growth 

 of the blastocyst is well initiated. 



In the Marsupial, notwithstanding the fact that the ovum 

 has become secondarily holoblastic, the mode of formation 

 of the blastocyst is essentially that of the Monotreme. 

 Cleavage is of the radial type, and owing to the persistence 

 of the shell, which with the zona forms a firm resistant 

 sphere enclosing the egg, the radially arranged blasto- 

 meres are able to assume the form of an open ring and to 

 proceed directly to the formation of the unilaminar wall of 

 the blastocyst. The enclosing sphere provides the necessaiy 

 firm surface over which the products of division of the upper 

 and lower cell-rings of the 16-celled stage can respectively 

 spread towards opposite poles, so as to directly constitute the 

 formative and non-formative regions of the blastocyst wall. 

 In my opinion it is the persistence of the resistant shell- 

 membrane round the ovum which conditions the occurrence 

 in the Marsupial of this direct method of blastocyst formation. 

 As in the Monotreme, so here also embryonal differentiation 

 commences oidy after the blastocyst has grown considerably 

 in size. 



In the Entheria, on the other hand, in the absence of the 

 shell-membrane, not only is the mode of formation of the 

 blastocyst quite different to that in the Marsupial, but 

 the relations of the constituent parts of the completed 

 structure also differ markedly from those of the homo- 

 genous parts in the latter. The cleavage pi'ocess here leads 

 only indirectly to the formation of the blastocyst, and must be 

 held to be cajnogeuetically modified as compared with that of 



