THE BIOLOGY OF THE CELL SURFACE 



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The egg of the starfish is interesting. Whilst other eggs, 

 as we have seen, have a very sharply limited period during 

 which fertilization is possible, the egg of this animal, whose 

 optimum period for fertilization is just after the dissolu- 

 tion of the germinal vesicle, is still capable of fertilization 

 at later stages of maturation and for a time after complete 

 maturation. But inasmuch as the fertilization-process 



c d 



Fig. 21. — Diagrams to illustrate the four fertilization classes (after Wilson). 



a, Class I; b, Class II; c, Class III; d, Class IV. 



either before or after maturation is below normal, this egg 

 belongs to Class 2. 



It is evident that an explanation of fertilization must 

 cover these four classes of eggs. A theory that covers 

 the fertilization of the sea-urchin egg and not that of eggs 

 fertilizable in the germinal vesicle stage and in subse- 

 quent stages of maturation would demand a separate 

 explanation for each of the other three classes. This in 

 turn would mean that fertilization diflFers in different eggs — 



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