94 J. P. HILL. 



the pei-ivitelline space and between the zona and sliell, is but 

 sb'glit. But as soon as the yolk becomes suiTounded by a 

 complete cellulai- membrane, i.e. as soon as the eg'g has 

 become converted into a thin-walled blastocyst, rapid growth 

 sets in, accompanied by the active imbibition of the nutrient 

 fluid, which is poured into the uterine lumen as the result of 

 the secretory activity of the abundantly developed uterine 

 glands. The fluid absorbed not only keeps the blastocyst 

 turgid, but it brings about tlie more or less complete dis- 

 integration of the yolk-mass, its constituent spherules 

 becoming disseminated in the fluid contents of the blastocyst 

 cavity. Although a distinct and continuous subgerminal 

 cavitv, such as appears beneath the embryonal region of the 

 Sauropsidan blastoderm, does not occur in the Monotreme 

 eo-g, vacuolar spaces filled with fluid develop in the Avhite 

 volk-bed underlying the site of the germinal disc and appear 

 to represent it. As Wilson and Hill remark ('03, p. 317), 

 " one can, without hesitation, homologise the interior of the 

 vesicle with the subgerminal cavity of a Sauropsidan egg, 

 extended so as to include by liquefaction the whole of the 

 yolk itself." In the Marsupial the blastocyst cavity has a quite 

 different origin, since it represents the persistent segmentation 

 cavity, whilst in the Eutheria the same cavity is secondarily 

 formed by the confluence of intra- or inter-cellular vacuolar 

 spaces, buc no one, so far as I know, has ever ventured to 

 assert that, because of this difference in mode of origin, the 

 blastocyst cavity in the series of the Mammalia is a non- 

 homogenous formation. 



To return to the matter under discussion, it appears to me 

 that the necessity which has arisen, consequent on the reduc- 

 tion in size of the ovum, for rapid growth of the same in 

 order to provide room for the development of an embryo and 

 for the storage of nutrient material furnished by the maternal 

 uterus, affords a satisfactory explanation of the much more 

 marked activity of the extra-embryonal region of the blasto- 

 derm as compared with the embryonal, which is such a striking 

 feature in the early ontogeny of the Monotremes, and not 



