The Development of the Alligator 265 
groove, mg in Figure 9/, into the shallow slit, pg, 
Figure 9m. 
There is no line of demarcation between the 
typical medullary groove region of Figure 9l and 
the equally typical primitive groove region repre- 
sented in Figure 9m. As was noted in the pre- 
ceding stage, the medullary folds are quite 
continuous with the folds of the primitive 
streak, and the medullary groove with the 
primitive groove; so that, unless we take the 
dorsal opening of the neurenteric canal as 
the point of separation, there is no line of 
division between these structures. The ento- 
derm (ev) and the lateral regions of the ecto- 
derm (ec) and mesoderm (mes) in Figure 9m are 
about as they were in Figure 9/, but in the mid- 
dle line is seen a compact mass of cells form- 
ing the primitive streak (ps), with the shallow 
primitive groove (pg) on the dorsal side. The 
cells on each side of the primitive groove and 
for a short distance below it are compact, as is 
shown in the figure, but as we pass ventrally 
and laterally they become looser and more an- 
gular to form the lateral sheets of mesoblast 
(mes), very much as is the case in the chick and 
other forms. For a few sections posterior to the 
one shown in Figure 9m the primitive streak may 
be seen, then it disappears, and only the ecto- 
derm and entoderm remain as thin sheets of 
tissue above the yolk. 
