The Development of the Alligator 257 
it be here so called, is proportionately more shallow 
than in the preceding figure and is actually much 
wider. The section passes behind the posterior 
end of the notochord, so that structure is not seen. 
Though not so well indicated as might be desired 
in the figure, the three germ layers are here indis- 
tinguishable in the middle line, and in the center 
of this mass of cells the blastopore (b/p) or neu- 
rentcric canal may be seen as a small vertical slit. 
As will be more fully described in the following 
stage, this canal opens dorsally a few sections 
posterior to the one under discussion and ventrally 
a few sections farther toward the head. 
In all the sections of this stage the ectoderm 
and entoderm are fairly thick in the region of the 
embryo proper, but become thinner until reduced 
to a mere membrane as we pass to more distal 
regions. Both layers are composed of loosely 
arranged cells, with scattered nuclei. Where the 
ectoderm becomes thickened to form the medul- 
lary folds, the cells are much more compactly 
arranged; hence this region stands out in strong 
contrast to the rest of the ectoderm. 
STAGE VI 
FIGURES 9a-9m (PLATES XIV., XV.) 
The embryo represented by this series of trans- 
verse sections is intermediate in development 
between those represented in surface views by 
17 
