The Development of the Alligator 239 
In Figure 3k there is no sign of the medullary 
groove, though the ectoderm (ec) is still much thick- 
ened in the middle line. The section passes, pos- 
terior to the notochord, through the anterior edge 
of the ventral opening of the blastopore (dlp). 
The mesoderm (mes) is here again continuous with 
the entoderm, around the edge of the blastopore, 
but is distinct from the ectoderm. 
Figure 3/ represents the third section posterior 
to the preceding. The blastopore, which passes 
upward and backward through the blastoderm, is 
seen as an enclosed slit (b/p). Itis surrounded by a 
distinct layer of compactly arranged cells contin- 
uous with the thickened ectoderm (ec) above, with 
the thin entoderm (ez) below, and laterally with 
the gradually thinning and scattering mesoderm 
(mes). 
Figure 3 is the next section posterior to the one 
just described. It passes through the dorsal 
opening of the blastopore (dlp), which appears as a 
deep, narrow cleft with thick ectodermal borders. 
The three germ layers are still continuous with 
each other, though the connection of the entoderm 
with the other two is slight. The sections poste- 
rior to this one will be described in the next stage, 
where they have essentially the same structure 
and are better preserved. 
Figures 37 and 30 are sagittal sections of an 
embryo of about the stage under discussion. In 
both figures the head-fold is seen as a deep loop 
