256 The Alligator and Its Allies 



Figure 8i shows the appearance of a section 

 through the mesoblastic somites, in one of which 

 a small myoccel (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 (nf) 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 o 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 



