THE EMBRYOLOGY OF THE OPOSSUM 59 



Figure 18 C, a more lateral view of the same section, shows 

 that the ectoderm covering the parietal mesoderm is continu- 

 ous with and indistinguishable from the extra-embryonic 

 ectoderm, which means that there is no such thing as an em- 

 bryonic area in the sense in which this term is ordinarily 

 used in mammalian embryology. The opaque light edge of 

 the 'embryonic area' seen in the whole mount (fig. 18 A, p.m.) 

 is the edge of the mesodermal thickening, not the ectodermal 

 thickening. This could not be determined from an external 

 view, but in sections it is obvious. After this stage it is easy 

 to see even in external views that the ectodermal thickening 

 is a medullary plate. The hearts, the limb ridge, and the 

 whole body wall form lateral to it. 



In describing his 3-somite specimen Selenka says (S. 125) : 



"Was aber den peripherischen Endsaum den "Keimwulst" 

 des Mesoderms betrifft, so zeigte sich hier nirgends die 

 mindeste Andeutung einer Entstehung von Mesodermzellen 

 in loco, noch eine Betheiligung des Ektoderms oder Ento- 

 derms; er markirte sich auf den Schnitten niemals als \Vulst 

 oder Anschwellung, und erst mil der Anlage des Sinus tenni- 

 nalis erscheint auch die peripherische Grenze des Mesoderms 

 bezw. des Gefassblattes, scharf abgesetzt. 



As pointed out in a previous paragraph the edge of the 

 spreading sheet of mesoderm is always sharply defined from 

 the very beginning, and the mesoderm definitely merges witli 

 the endoderm along this line. This line is the mesodermal 

 rim, marked m.r. in figure 18 A. Whether any mesodermal 

 cells are actually arising from the endoderm is difficult to de- 

 termine, but at any rate endoderm and mesoderm are com- 

 pletely fused along this line, and many mitoses are found in 

 the region of fusion. 



Stage 23 



The coelomic cavities and the subcephalic fold. I have 

 only one specimen upon which to base a description of stage 

 23. This specimen (17181) was a retarded member of a litter, 

 the other members of which belonged to stage 24. Also it 



