288 The Allirator and Its Allies 



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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 13^ is about one hundred and twenty- 

 five sections posterior to Figure 1 3c. Although not 

 so small as the sections that follow it, this section 

 is considerably smaller in area than the one last 

 described. The amnion {a), 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 {nt) is not only relatively but actually larger 

 than in the more anterior regions. Beneath the 

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



