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Fig. 2. Inhibition of cleavage of demembranated, fertilized eggs of the 

 sea urchin Lytechimts pictus by treatment with a rabbit antiserum against 

 fertilizin of Strongylocentrotus purpuratus. Figures 2a to f are of eggs in 

 antiserum. Figures 2a' to f are of eggs in control serum. The eggs were 

 placed in the antiserum and control serum at 10 min after insemination 

 and photographs taken at following times (at 17° C) after fertilization: 

 a,a', 2 hr; b,b', 2^2 hr; c,c', 3 hr; d,d', 4^2 hr; e,e', 6 hr; f,f', 24 hr. Magnifica- 

 tion, lOOx. 



