The Development of the Alligator 259 



rounds the embryo as an irregular membrane of 

 some thickness in which no arrangement into 

 layers can be seen. The epidermal ectoderm (ep) 

 is composed of the usual loosely arranged cells, 

 so that it is clearly distinguishable from the com- 

 pactly arranged cells of the nervous layer (nl) , from 

 which it is separated by only a line. 



In Figure qb, which shows a section a short dis- 

 tance posterior to the preceding, the medullary 

 canal (me) is somewhat deeper and is still open 

 ventrally. There is a distinct space between 

 the nervous (;;/) and epidermal (ep) layers of the 

 ectoderm, in which space a few mesoblast cells 

 (mes) may be seen. The section is cut just poste- 

 rior to the edge of the amnion, so that there is now 

 neither amnion nor yolk above the embryo. 



Figure gc is about ten sections posterior to Figure 

 96. The section passes through the anterior wall 

 of the bent-under part of the medullary canal 

 (me') , so that the actual canal is shown only on the 

 dorsal side (me), where it is completely closed and 

 begins to assume the shape of the typical embry- 

 onic spinal cord. The space between the super- 

 ficial (ep) and nervous (/) layers of the ectoderm is 

 quite extensive and is largely filled by a fairly 

 compact mass of mesoderm (mes). 



Figure yd, although only five sections posterior 

 to the preceding, shows a marked change in struc- 

 ture. The medullary canal (me) is here of the 

 typical outline for embryos of this age. A large, 



