268 The Alligator and Its Allies 



noted in the preceding stage has advanced so far 

 that a considerable part of the embryo now pro- 

 jects forward under the blastoderm. In some 

 cases it is almost concealed in a dorsal view ; in other 

 cases it may easily" be seen through the transparent 

 membranes, especially after clearing. 



In opening eggs of this stage one is at first apt 

 to underestimate the size of the embryos, since 

 the anterior part of the embryos cannot be seen 

 until after they are removed from the yolk and are 

 viewed from the ventral side. 



The embryo from which the series of transverse 

 sections of this stage was made, while of the same 

 state of development as that shown in Figure 1 1 , 

 was more fully covered by the blastoderm than is 

 shown in the surface view in question. 



Figure na passes through the tip of the head. 

 Dorsal to the embryo is the ectoderm and a thick 

 mass of yolk (y). The amnion (a) is seen as an 

 irregular membrane which entirely surrounds the 

 head. The medullary canal (me) is entirely closed 

 except at the extreme anterior end, which is bent 

 downward so that the opening is on the ventral 

 side. The nervous (nl) and epidermal (ep) layers 

 of the ectoderm are in contact throughout, but 

 are clearly distinguishable because of the difference 

 in the compactness of their cells. 



In Figure i ib is represented a section, behind the 

 preceding, which passes through the posterior tip 

 of the turned-under anterior end (me'}. Here the 



