DEVELOPMENT OF THE OPOSSUM 53 



innermost cells in the last mentioned stage are clearly primitive 

 entoderm. From the margins of the discs entodermal cells may 

 be seen migrating out beneath the wall of the blastocyst. There 

 can be no doubt from this brief statement, that entoderm forma- 

 tion proceeds in the opossum very similarly to the method 

 obtaining in the Australian marsupials. Further details will now 

 be mentioned. 



The youngest blastocysts to give any indication of cell pro- 

 liferation as just described are those of batch No. 144 (fig. 27). 

 The blastocysts contain somewhat over 100 cells. Very little 

 albumen has been absorbed and the cells of the non-formative 

 region have not become much flattened; hence, these specimens 

 are only a little advanced over the 50- to 60-celled blastocysts 

 described in the preceding section. In figure 27, which has 

 formative cells on the reader's left, a large cell is dividing and 

 other cells proliferating from the wall at other points. These 

 latter appear in the figure as mere included cells, but a study of 

 the series of sections shows them to be attached to the wall. 

 The specimens, though poorly fixed, are introduced here because 

 they are clearly somewhat advanced in the direction of the 

 older blastocysts now to be described. These latter are well 

 fixed and for the most part excellently stained preparations. 



Specimen No. 43 (9) shows the fewest entodermal cells. 

 Though somewhat crushed at right angles to the edge of the 

 microtome blade (text fig. 6 F) it is otherwise well preserved and 

 the cellular elements are perfectly stained. A portion of the 

 blastocyst wall reproduced in figure 28 shows an 'entodermal 

 mother cell' (ENT'^) in the wall and also a primitive entodermal 

 cell {ENT), both stained darker than the other cells of the 

 blastocyst. The former cell shows a tendency to pull from its 

 place in the wall in precisely the manner illustrated by Hill in 

 his figure 75, Plate 8. In surface view this cell {ENT^ fig. 28) 

 would appear underneath the adjoining cell of the blastocyst 

 wall. 



More cell elements are seen in specimen 43 (8), figures 29 and 

 30. The primitive entoderm cells {ENT in the figures) are 

 mostly in close contact with the ectodermal cells of the wall. 



