lis J. p. HILL. 



should be recollected, which exhibit the greatest growth- 

 energy during the formation of the blastocyst in the Mono- 

 treme and Marsupial, and so their greater activity in the 

 Eutherian morula is only what might be expected. Dividing 

 7nore rapidly than the formative cells, they gradually grow 

 round the latter, and eventually form a complete outer layer 

 enveloping the inert formative cell-group. This process ofovei'- 

 growth or epiboly is entirely comparable in its effect Avith the 

 spreading of the extra-embryonal region of the unilaminar 

 blastodermic membrane in the Monotreme to enclose the yolk- 

 mass, and with that of the non-formative cells in the Marsupial 

 to complete the lower hemisphere of the blastocyst, growth 

 round an inert central cell-mass being here substituted for 

 growth over the inner surface of a resistant sphere constituted 

 hj the egg-envelopes, such as occurs during the formation of 

 the blastocyst in the Monotreme and Marsupial. Just as the 

 first objective of the cleavage process in the latter is to effect 

 the completion of the cellular wall of the blastocyst, so here 

 the same objective recurs, and is attained in the simplest 

 possible way in the new circumstances, viz. by the rapid en- 

 velopment of the formative by the non-formative cells. Thus 

 at the end of the cleavage process in the Eutherian we have 

 formed a solid entypic morula in which an inner mass of 

 formative cells is completely surrounded by an outer envelop- 

 ing layer of non-formative or tropho-ectodermal cells, homo- 

 genous with the extra-embryonal ectoderm of the Sauropsidan 

 and Monotreme and the non-formative region of the uni- 

 laminar blastocyst of the Marsupial. Conversion of the solid 

 morula into a hollow blastocyst capable of imbibing fluid 

 from the uterus and of growing rapidly now follows. Inti-a- 

 or intercellular vacuoles appear below the inner cell-mass, by 

 the confluence of which the blastocyst cavity is established, 

 and the inner cell-mass becomes separated from the envelop- 

 ing layer of tropho-ectoderm, except over a small area where 

 the two remain in contact. 



The complete enclosure of the formative cells of the inner 

 cell-mass by the non-formative ectodermal cells of the 



