256 The Alligator and Its Allies 
Figure 82 shows the appearance of a section 
through the mesoblastic somites, in one of which 
a small myocoel (myc) is seen. As is seen by the 
size of the figure, which is drawn under the same 
magnification as were all the sections of the series, 
the embryo in this region is much smaller in section 
than it is toward either end, especially toward 
the anterior end. The medullary groove (mg) is 
still more shallow than in the more anterior sec- 
tions, and the ectoderm (ec), with which its folds 
are continuous laterally, is here nearly horizontal. 
The mesoblast (mes) is of a more compact nature 
than in the preceding section and shows little or no 
sign of cleavage, although a distinct myoccel may 
be seen and cleavage is well marked in sections 
between this one and the preceding. 
_ The notochord (nt) has about the same appear- 
ance as in the preceding section, but is more 
distinctly separated from the surrounding cells. 
Figure 87 is through the posterior end of the 
embryo; it shows the relation of parts in the region 
of the primitive streak. Although not visible in 
surface views, and hence not represented in Fig- 
ure 8, the medullary groove is continued without 
any line of demarcation into the primitive groove, 
and the medullary folds into the edges of the primi- 
tive streak, so that it is impossible to set any 
definite boundaries between these structures unless 
the dorsal opening of the blastopore be taken as the 
point of division. The medullary groove (mg), if 
