240 The Alligator and Its Allies 
of ectoderm and entoderm, while the head-fold of 
the amnion is seen at a. 
The beginning of the foregut is seen in Figure 3n 
(fg), which is the more nearly median of the two 
sections, Figure 30 being a short distance to the 
side of the middle line. 
In Figure 30 the thin entoderm (en) is separated 
from the much thicker ectoderm (ec) by a consider- 
able layer of rather loose mesoderm (mes). In 
Figure 37, which is almost exactly median in posi- 
tion, there is, of course, no mesoderm to be seen 
in front of the blastopore, and the entoderm 
shows a considerable increase in thickness, due to 
the formation of the notochord (é). The blasto- 
pore (1p) is the most striking feature of the figure, 
and is remarkable for its great width in an antero- 
posterior direction. Its anterior and posterior 
borders are outlined by sharply defined layers of 
ectoderm and entoderm. Posterior to the blasto- 
pore the lower side of the ectoderm is continuous 
with a considerable mass of cells, the primitive 
streak (ps). 
STAGE III 
FIGURES 4, 42, 5, 5@, AND 6-67 (PLATES X., XI.) 
“Figures 4 and 4a are of an embryo removed, on 
June 18th, from an egg which had been taken out 
of an alligator two days before. Figure 4, a dorsal 
view, is of special interest in that it shows a second- 
