DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 21 



Figure gc is about ten sections posterior to figure gb. The section 

 passes through the anterior wall of the bent-under part of the medul- 

 lary canal ( mc'), so that the actual canal is shown only on the dorsal 

 side {mc), where it is completely closed and begins to assume the 

 shape of the typical embryonic spinal cord. The space between the 

 superficial (ep) and nervous (nl) layers of the ectoderm is quite 

 extensive and is largely filled by a fairly compact mass of mesoderm 



())ICS) . 



Figure gd, although only five sections posterior to the preceding, 

 shows a marked change in structure. The medullary canal (mc) is 

 here of the typical outline for embryos of this age. A large, com- 

 pact mass of cells (cut) 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 depression of the ectoderm (cc) and entoderm (en) of the blas- 

 toderm 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 epidermal ectoderm is here continuous with 

 the thin layer of superficial ectoderm (ec) of the blastoderm, while 

 the entoderm (en) of the blastoderm is still continuous 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 gf is about twenty sections posterior to the preceding sec- 

 tion, and passes through the point of separation of the folds of the 

 entoderm (en). From this point the entoderm gradually flattens 

 out, leaving the enteron unenclosed. The medullary canal (mc) 

 and notochord (nt) are about as in the preceding section, but the 

 ectoderm (ep) is somewhat thinner and more flattened. The meso- 

 derm (mes) on the right side exhibits a distinct cleavage, the result- 

 ing body cavity (be) being a large, triangular space. 



Figure gg, the twenty-fifth section posterior to that represented in 



