260 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 ge 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 (ez) 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 of is about twenty sections posterior to 
