
No. 427.] | THE EGG OF BUFO LENTIGINOSUS. 543 
lateral sheets of mesoderm by a process of splitting off from 
the outer yolk cells has been noted by Schwink (26) in Bufo 
vulgaris, by Scott and Osborn (23) and by Jordan in the newt, 
and by Morgan in the frog. 
At the close of the blastula stage a very large segmentation 
cavity is found in the upper hemisphere of the egg directly 
under the black pole (Figs. 9, 11, 12). During the formation 
of the archenteron this segmentation cavity decreases con- 
siderably in size, and is pushed out of its position, coming to 
lie below the archenteron and being separated from it by only 
a thin layer of cells (Fig. 16, S.C.). During the closure of 

Fic. 18. — Outline of the entire section from which Fig. 17 was drawn. 
the blastopore the archenteron increases greatly in size, and 
there is then usually but a single layer of large yolk cells 
between it and the segmentation cavity (Fig. 19). When the 
blastopore is nearly closed, I have frequently found eggs in 
