DEVELOPMENT OF THE OPOSSUM 57 



Included yolk and coagulum are still seen in several of the 

 specimens, and large yolk granules, such as observed regularly 

 in the earlier stages, still occur within the blastomeres (figs. 29, 

 32 and 33). Inasmuch as the thickened area of these blasto- 

 cysts is very clearly the formative region, the question arises: 

 which portion, the thicker or the thinner half, of the earlier 

 (50-celled) blastocysts is formative and which the non-formative 

 portion? Polar differentiation in the opossum, it will be recalled, 

 first manifests itself by a more rapid thinning at one pole of the 

 blastocyst of 50 or more cells. It will be recalled also, that 

 Hill, interpreting Selenka's two earliest blastocysts in the light of 

 the cleavage process in Dasyurus, considers the larger, more yolk- 

 laden, cells of the lower pole as non-formative; the other as forma- 

 tive. I venture the assertion that the thicker-walled pole is the 

 formative region (compare text figs. 5 and 6). The thinning 

 of half or more of the blastocyst is simply the accompaniment of 

 the differentiation of the embryonic disc. Blastocysts of No. 

 144, although shrunken, are especially instructive in this con- 

 nection, since several of them are clearly a little more advanced 

 than those described in the last section. From figure 27 and 

 text figure 6 A, the change here indicated may well be seen. 

 One region of the blastocyst (to the right in the figures) is more 

 attenuated and larger in extent than the other. The differen- 

 tiation has gone further than in any blastocyst shown in figure 

 5, and figure 27 is only a little more advanced. The whole 

 series is so nearly complete as to justify the conclusion that, 

 once begun, the differentiation Of the formative region is pro- 

 gressive. 



From the specimens described in this section, therefore, there 

 can be no doubt whatever that, in common with other mar- 

 supials, the opossum lacks the morula stage in its development. 

 It is furthermore clear that cell proliferations do occur from the 

 blastocyst wall in the region of the blastodisc and that these 

 proliferations result in the formation of the definitive entoderm. 



