56 EDWARD MCCRADY, JR. 



Being the border of what will later become the area vascu- 

 losa, the prospective site of origin of the sinus terminalis, 

 and a region of fusion of mesoderm and endoderm, the' meso- 

 dermal rim has much in common with the 'germ wall' of 

 avian eggs. The absence of yolk in the opossum vesicle at 

 this time, however, makes the question of their homology 

 difficult to decide. 



At this stage the primitive groove attains its greatest 

 relative and absolute length, and simultaneously a clear spot 

 appears at its anterior tip. The vesicle is now 3.2 mm. in 

 diameter. 



Stage 21 



The notochord and the medullary groove. The clear spot 

 in stage 20 is due to the shortening of the primitive streak 

 at its anterior end. When the vesicle is about 3.7 mm. in 

 diameter (specimen 17179) this shortening at the anterior 

 end is going on more rapidly than the growth at the pos- 

 terior end, so that the primitive streak is decreasing in total 

 length. The shortening process is correlated with the ap- 

 pearance of two new structures the notochord and the 

 medullary groove for it seems to be due to a tangential 

 split in the substance of Hensen's node. The lower or more 

 medial cells are separated from the more superficial cells 

 and become the notochord. This leaves only a thin sheet 

 of medullary plate in the middle line above the notochord, 

 which, correspondingly, when illuminated from below ap- 

 pears as a light streak. The medullary plate sinks in 

 slightly along this thin line and thus forms a medullary 

 groove which is anterior to the primitive groove. 



During all this time the medullary plate is constantly in- 

 creasing in length. Most of this increase seems to be due 

 to the multiplication of cells at the posterior growing tip; 

 but part of it may be due to migration of cells, for the plate 

 is also narrowing in width at the same time that it is in- 

 creasing in length and I see no indication of changes in cell 

 shape which might account for this. 



