GRAY CRESCENT 



TOP VIEW 



SIDE VIEW 



FIRST CLEAVAGE 



COMPLETE EMBRYOS 



Fig. 23. The significance of the gray crescent as revealed by separating the 

 cells at the two-cell stage. Left: The gray crescent is shown in top view. When 

 this egg divides, one half of the gray crescent passes into each of the two 

 cells. If these cells are separated from each other each cell will form a com- 

 plete embryo. Right: A side view of the gray crescent and a first cleavage in 

 which one cell contains all of the crescent, while the other cell lacks any part 

 of it. If these cells are separated, the one lacking the crescent fails to develop. 



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