The Development of the Alligator 263 



embryo, therefore, through the posterior one fourth 

 of its length. The chief difference between this 

 and the preceding section is in the medullary canal, 

 which is here open and is in the form of a wide 

 groove with an irregular, rounded bottom and 

 vertical sides. The size of the section is consider- 

 ably greater than in the preceding, the increase 

 being especially noticeable in the notochord (nt), 

 which is cut near its posterior end. There is little 

 or no sign of mesoblastic cleavage. 



Figure 97 is about twenty sections posterior 

 to Figure gi. The medullary groove (mg) is con- 

 siderably larger than in the more anterior regions, 

 and its folds are somewhat inclined toward each 

 other, though still wide apart. The notochord 

 and entoderm are fused to form a large, compact 

 mass of tissue close under the ventral wall of the 

 medullary groove. On the ventral side of this 

 mass of cells a groove (Up) marks the anterior and 

 ventral opening of the blastopore shown in the next 

 figure. The mesoblast shows no sign of cleavage. 



Figure qk shows the medullary groove (mg) 

 in about the same position as in the preceding 

 section. The blastopore (blp) is here seen as a 

 small cavity in the center of the large mass of cells 

 that was noted in the last figure. The entoderm 

 (en) is continuous from side to side, but is not so 

 sharply differentiated from the other germ layers 

 as is represented in the figure. 



Figure gl is four sections back of the preceding; 



