DEVELOPMENT OF THE AMERICAN' ALLIGATOR — REESE 23 



in the notochord ( nt), which is cut near its posterior end. There is 

 little or no sign of mesoblastic cleavage. 



Figure gj is about twenty sections posterior to figure gi. The 

 medullary groove (mg) is considerably larger than in the more an- 

 terior 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 (blp) marks the anterior and ventral opening of the 

 blastopore shown in the next figure. The mesoblast shows no sign 

 of cleavage. 



Figure gk 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 9/ is four sections back of the preceding; the wide, dorsal 

 opening (blp) of the blastopore or neurenteric canal into the medul- 

 lary groove (mg) is shown. The blastopore or neurenteric canal, 

 then, is still at this stage a passage that leads entirely through the 

 embryo, the medullary canal being in this region unenclosed above. 

 Ventrally it is seen as a narrow opening through the entoderm ; it 

 then passes upward and backward, behind the end of the notochord, 

 as a small but very distinct canal, which may be traced through about 

 ten sections. The enclosed portion of the canal lies, as has been 

 stated (figure gk, blp), in the center of the mass of cells that is fused 

 with or is a part of the floor of the medullary groove. 



The above-described neurenteric canal is essentially like that de- 

 scribed by Balfour (2) in the lace rt Hi a, He does not say, however, 

 and it is not possible to tell from his figures, whether there is a long, 

 gradually diminishing groove posterior to the dorsal opening of the 

 canal, as in the alligator. He says that the medullary folds fuse 

 posteriorly until the medullary canal is enclosed over the opening of 

 the neurenteric canal ; also that "the neurenteric canal persists but a 

 very short time after the complete closure of the medullary canal." 



In figure gm, for about thirty sections (one-tenth the entire length 

 of the embryo), behind the section represented in the last figure, 

 there is a very gradual change in the embryo, converting the deep 

 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 9/ and the equally typical primitive groove 



