The Development of the Alligator 265 



groove, mg in Figure 9/, into the shallow slit, pg, 

 Figure gm. 



There is no line of demarcation between the 

 typical medullary groove region of Figure gl and 

 the equally typical primitive groove region repre- 

 sented in Figure gm. As was noted in the pre- 

 ceding stage, the medullary folds are quite 

 continuous with the folds of the primitive 

 streak, and the medullary groove with the 

 primitive groove; so that, unless we take the 

 dorsal opening of the neurenteric canal as 

 the point of separation, there is no line of 

 division between these structures. The ento- 

 derm (en) and the lateral regions of the ecto- 

 derm (ec) and mesoderm (mes) in Figure 977? are 

 about as they were in Figure gl, but in the mid- 

 dle line is seen a compact mass of cells form- 

 ing the primitive streak (ps), with the shallow 

 primitive groove (pg) on the dorsal side. The 

 cells on each side of the primitive groove and 

 for a short distance below it are compact, as is 

 shown in the figure, but as we pass ventrally 

 and laterally they become looser and more an- 

 gular to form the lateral sheets of mesoblast 

 (mes), very much as is the case in the chick and 

 other forms. For a few sections posterior to the 

 one shown in Figure gm the primitive streak may 

 be seen, then it disappears, and only the ecto- 

 derm and entoderm remain as thin sheets of 

 tissue above the yolk. 



