540 THE AMERICAN NATURALIST. [VoL. XXXVI. 
of the dorsal wall of the archenteron receives additions from the 
yolk, as believed to be the case by Assheton and by Wilson, I 
have not been able to determine. 
After the ventral lip of the blastopore has formed and the 
blastopore is beginning to close (Fig. 6), a division of the 

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Fic. 15. 
Lettering as in previous figures. 
ectoderm into two layers is first noticed. A median sagittal 
section at this period (Fig. 15) shows an outer ectodermal wall 
composed of a single layer of small, deeply pigmented cells 
which are arranged very regularly. The inner ectodermal 
sheath is composed of two or three layers of much larger cells 

