FIG. 15 



A, egg of the sea-urchin, Paracentrulus Jividus (from Boveri), before its matura- 

 tion of fertilization, uniformly pigmented; B, the same just after formation of 

 the polar bodies (p.); C, the same slightly later (in this ease just after entrance 

 of the sperm at s.p. ) when ready for fertilization, with pigment localized in a 

 sub-equatorial belt; D, the 2-cell stage. 



