DEVELOPMENT OF THE OPOSSUM 37 



7-celled stages the tendency of the cells to migrate toward the 

 periphery and to become closely appressed to the albumen layer 

 is very apparent. One 12-celled egg is an a-lmost fully-formed 

 blastocyst (text fig. 5 ^); the cells are very flat and closely 

 applied to the albumen, precisely as in eggs of more than double 

 the number of cells. On the other hand, in a number of 8- to 

 16-celled eggs, not all of the blastomeres touch the albumen 

 layer and in these cases they maintain their spherical shape 

 (fig. 21). Attention should be called at this point to a 16-celled 

 egg, 85 (12), of which a model was made. This egg is unique for 

 specimens of this batch in that it contains an included cell some 

 distance removed from the peripheral cells making -up the wall 

 of the 'blastocyst' and so situated as to make it improbable that 

 it could ever have attained a place in the wall of the blastocyst. 

 This egg will be referred to again. 



b. The eliminated yolk 



The processes in the development of the opossum egg up to 

 the 16-celled stage described above fully answer the question 

 propounded by Selenka as to the manner in which the yolk 

 reaches the cavity of the blastocyst. To quote: 



Auf welche Weise die in Figure 8, 10 and 11 mit i bezeichneten Dotter- 

 ballen in die Furchungshohle gelangen, weiss ich nicht zu sagen; in 

 d^n ersten bsidsn Stadien dsr Furchimg fehlen sie ganz, ebenso sach 



Fig. 5 Sections through early unilaminar blastocysts (X 250). i, gap in wall 

 of lilastocyst. 



A, early attempt at blastocyst formation in 12-celled egg No. 144 (1) b. B, 

 31-celled egg, No. 18 (18), with only half of blastocyst formed normally. C, 

 No. 88 (1), the only blastocyst here figured which is entirely devoid of included 

 cells. D, No. 50 (4), 50-celled blastocyst with almost uniform wall; this speci- 

 men has several included cells, not shown in this section. E, No. 88 (2), shows 

 polar differentiation in its greater thickness of cells at one pole as compared 

 with cells at the opposite pole (Cf. G, and others). F, No. 88 (14), showing 

 excessive proliferation from cells in wall of blastocyst (Cf. / and L). G and N, 

 two sections of No. 88 (4). H and I, two sections of No. 88 (7). J and M, two 

 sections from opposite poles of No. 88 (6) ; note large included blastomere in 

 J. K and L, two sections of No. 88 (16) ; note large cell in wall of K, and included 

 cells in L. 0, more advanced blastocyst, No. 144 (1) a; transition betweeri the 

 series shown in this figure and those shown in figure 6. 



