The Development of the Alligator 235 
mentioned, where, medially, the ectoderm, meso- 
derm, and entoderm form a continuous mass of 
cells. Laterally the mesoderm (mes) is a distinct 
layer of cells of a fairly characteristic mesodermal 
type. The notochord is not yet discernible, 
though a slight condensation of cells in the middle 
line may indicate its position. 
Figure 2f is one of the four sections that were cut 
through the blastopore (dlp), which is a hole of 
considerable size that opens, as the figure shows, 
entirely through the blastoderm. Along the walls 
of the blastopore the ectoderm and entoderm are, 
of course, continuous with each other and form 
a sharply defined boundary to the opening. As 
we pass laterally from the blastopore the cells 
become less compact, and are continued on each 
side as the mesodermal layer (mes). In this series 
the sections posterior to the blastopore were 
somewhat torn, and so were not drawn; but they 
probably did not differ materially from those 
of the corresponding region of the immediately 
following stages, which are shown in Figures 3m 
and 67 and will be described in their proper order. 
STAGE II 
Ficures 3-30 (PLate VII., VIII., IX.) 
The next stage to be described is shown in sur- 
face views in Figures 3 and 3a. Of this stage Clarke 
says: 
