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 11, 
was more fully covered by the blastoderm than is 
shown in the surface view in question. 
Figure I1a 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 (mc) is entirely closed 
except at the extreme anterior end, which is bent 
downward so that the opening is on the ventral 
side. The nervous (n/) 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 11) is represented a section, behind the 
preceding, which passes through the posterior tip 
of the turned-under anterior end (mc’). Here the 
