DEVELOPMENT OF THE OPOSSUM 61 



sists of a formative and a non-forniative portion. This inter- 

 pretation homologises the opossum egg with that of Dasyurus, 

 in which polar differentiation is structurally evident throughout 

 early cleavage (text figs. 1, 3, 4 and 6, pages 39-40). 



13) Since the blastocyst of the opossum is already anticipated 

 in the 16-celled stage by the arrangement of the cells into a 

 hollow sphere, the blastocyst is completed at an early stage, 

 that is, in eggs of about 40 cells. By this time the ovum has 

 grown very little, the albumen layer being as thick as in previous 

 stages. The corresponding stage of Dasyurus contains about 

 125 thin cells which lie immediately against the shell membrane. 

 About this time polar differentiation is manifested for the first 

 time in the history of the opossum egg as a thinning of the cells 

 at the pole destined to be the extra-embryonic area of the defini- 

 tive vesicle. Included cells are almost always present in the 

 early blastocysts (figs. 4, 5, 6, 26, 27, pages 42 and 49). 



14) The morula stage is absent in the opossum egg. The ento- 

 derm is formed by the proliferation of specialized 'entoderm 

 mother cells' which appear only in the embryonic area. The 

 entoderm mother cells may or may not migrate from their posi- 

 tion in the wall of the blastocyst, but in either case they give off 

 by division primitive entodermal cells which spread out at first 

 immediately beneath the embryonic ectoderm and later beyond 

 this area, until they completely line the entire vesicle. Ento- 

 derm formation sets in when about half the albumen is absorbed 

 and the blastocyst is 0.6 mm. or less in diameter. Hence there 

 is no such great period of growth as obtains in Dasyurus. The 

 didermic blastocyst of the opossum is completed when the egg 

 is only about double the original size as measured through the 

 shell membrane (text fig. 4, fig.s. 28-38, pages 49-57). 



