THE EMBRYOLOGY OF THE OPOSSUM 41 



cells in the immediately preceding stages, so that they neces- 

 sarily have attained this size by growth, not by remaining un- 

 divided. For instance, in specimen 16218 there is a single 

 endoderm mother-cell the longest diameter of which is 60 p, 

 the shortest 42 p. There has not been a cell of this size in 

 the egg since the two B-cells of the 6-celled stage divided. The 

 average endoderm mother-cell in the early stages of endo- 

 derm formation is about 46 M X 35 p in its longest and shortest 

 diameters respectively. This is about the size of the four 

 large A-cells in the 8-celled stage. The last of these four is 

 present in the 11-celled egg. From the 12-celled stage until 

 the first endoderm mother-cell appears (50- to 60-celled stage) 

 the cells become progressively smaller. Accordingly, it is 

 clear that the large size of the endoderm mother-cells is due 

 to a rather sudden and exceptional growth on their part. 



Being hemmed in laterally by the surrounding cells and 

 peripherally by the zona pellucida, which is still distinctly 

 visible though thin, the endoderm mother-cell as it enlarges 

 extends itself principally in a median direction. In other 

 words, it juts conspicuously into the blastocoel. When it at- 

 tains its maximal size it usually rolls out of the protoderm 

 (the primitive pluripotential layer from which all the germ 

 layers are derived) and into the blastocoel. Sometimes, how- 

 ever, it divides first; and occasionally two divisions occur be- 

 fore the base of the original cell loses contact with the cells 

 remaining at the surface. This produces for a time a chain 

 of cells projecting into the blastocoel. But in any case, the 

 cells soon free themselves from the protoderm altogether. 

 When free in the blastocoel these endoderm mother-cells are 

 perfectly spherical. They show no consistent peculiarities 

 except those of size and the habit of leaving the blastocyst 

 wall. Some, but not all, stain a little darker than the other 

 cells. 



The gap left at the surface by the inward migration of this 

 large cell is soon effaced. Selenka considered such a gap the 

 homologue of the blastopore; and though he recognized that 

 the gap itself is very transitory (1887, p. 114) he described 



