26o The Alligator and Its Allies 



compact mass of cells (ent) appears at first glance 

 to be the same that was noted in the preceding 

 stage at the tip end of the turned-under medullary 

 canal ; it is, however, the extreme anterior wall of 

 the enteron,which is in close contact with the above- 

 mentioned tip of the medullary canal. Between 

 this anterior wall of the enteron, of which wall it is 

 really a part, and the medullary canal is the noto- 

 chord (nt). The space surrounding the notochord 

 and enteron is filled with a fairly compact mass 

 of typical, stellate mesoblast cells. The depres- 

 sion of the ectoderm (ec) and entoderm (en) of the 

 blastoderm caused by the formation of the head- 

 fold is here less marked, and the dorsal side of the 

 embryo in this region is slightly elevated above 

 the level of the blastoderm. 



Figure qe represents a section passing through 

 the posterior edge of the head-fold. The epider- 

 mal ectoderm is here continuous with the thin layer 

 of superficial ectoderm (ec) of the blastoderm, while 

 the entoderm (en) of the blastoderm is still contin- 

 uous beneath the embryo. The thick ectoderm of 

 the embryo is sharply differentiated from the thin 

 layer of ectoderm that extends laterally over the 

 yolk. The pharynx (ent) is a large cavity whose 

 wall is thick except at the dorsal side, where it is 

 thin and somewhat depressed, apparently to make 

 room between it and the medullary canal for the 

 notochord (nt). 



Figure 9/ is about twenty sections posterior to 



