The Development of the Alligator 273 
similar to that shown in Figure 11k, which is about 
the twentieth section posterior to Figure 117. The 
break (ec’) in the ectoderm is here seen as a com- 
pact group of cells which at first glance seem to be 
continuous with a rounded mass of cells below (ps). 
Examination under greater magnification, how- 
ever, shows that the two groups of cells are distinct. 
As the sections are followed back of this region, the 
upper mass of cells (ec’) gradually disappears, and 
after its disappearance the lower mass (ps), which 
is already continuous with the mesoderm (mes) on 
either side, becomes continuous with the under 
side of the ectoderm. The mass of cells (ps) is 
apparently the primitive streak, though it is dis- 
tinct from the ectoderm for a considerable distance 
posterior to the neurenteric canal. Just what 
may be the meaning of the thickened ridge of 
ectoderm (ec) it is difficult to determine. 
STAGE IX 
FIGURES 12-12g (PLATES XVIII., XIX.) 
The entire length of the embryo proper is 6.5 
mm. from the extreme posterior end to the region 
of the midbrain (v?), which now, on account of the 
cranial flexure, forms the most anterior part of 
the body. Besides being slightly longer than the 
preceding stage, the embryo has increased in thick- 
ness, especially in the anterior region, where the 
enlargement of the cerebral cavity is considerable. 
18 
