452 



ZOOLOGY 



of the cleavage stage the white or yolk side of the egg becomes 

 enveloped by the black dividing cells, excepting for a small 

 white plug edged by a sharp groove. This is where ceUs have 





( ' 



H 



Fig. 408. — Cleavage of froe'.s ess, A-C ; H, beginning (if gastrulation ; 

 I, the white plug. From Morgan, "Development of the Frog's Egg." 



