288 The Alligator and Its Allies 



this section, is nearly as large in cross-section as all 

 the rest of the embryo. As seen in such a section 

 it is entirely detached from the body of the embryo, 

 and in this particular case has about the shape of 

 the human stomach. The mesoblastic portion 

 of its wall (mes'} is of very irregular thickness; it 

 forms a dense layer entirely around the outside, 

 except for the pointed dorsal region, and is espe- 

 cially thick along the ventral margin, where it is 

 thrown into well marked folds, the heavy muscle 

 columns. Lining the cavity of the heart is the 

 membranous endothelium (en'}, and between this 

 and the dense outer wall just described is a loose 

 reticular tissue with but few nuclei. 



As the series is followed toward the tail the 

 sections diminish in size until, at a point about 

 one third the embryo length from the posterior 

 end, they are of scarcely one fourth the area of the 

 sections through the region of the hindbrain. 



Figure i sd is about one hundred and twenty- 

 five sections posterior to Figure 13^. Although not 

 so small as the sections that follow it, this section 

 is considerably smaller in area than the one last 

 described. The amnion (), which was not repre- 

 sented in the last three figures, is very evident 

 here. The spinal cord (sc} is considerably smaller 

 here than in the preceding figure, while the noto- 

 chord (?//) is not only relatively but actually larger 

 than in the more anterior regions. Beneath the 

 notochord is the aorta (ao), now a single large 



