THE PROBLEM OF ACTIVATION 265 



of cortical material present. With a minimum of cor- 

 tical material a fertilization reaction occurs and nuclear 

 division may follow, but in such a case the cytoplasm 

 does not divide. 



The writer (1914) has previously urged the possi- 

 bility that the spermatozoon undergoes some modifica- 

 tion, necessary for its part in the internal events of 

 fertilization, in its passage through the cortex of the 

 egg, possibly by union with fertilizin or other substance 

 of the cortex; in other words, that "the spermatozoon 

 needs itself to be fertilized." 



The material of the sperm nucleus is in a different 

 physiological state from the egg nucleus. In the cases 

 in which the sperm enters the egg during maturation 

 the egg nucleus is concerned in the maturation divisions; 

 in those cases (e.g., sea urchins) in which maturation 

 is completed before penetration of the spermatozoon 

 the egg nucleus is in the resting, interkinetic, vesicular 

 state, while the sperm nucleus is in the state of greatest 

 condensation of the chroma tin; but in either case the 

 sperm nucleus is associated with cytoplasm which has 

 received the cortical activation and which may hence 

 be regarded as more reactive. It is therefore natural 

 that the karyokinetic phenomena, which constitute the 

 normal reaction of the egg, should center around the 

 sperm nucleus, and the formation of the sperm aster 

 may be regarded as the first step in this process. The 

 cytological study of artificial activation has shown that 

 the cytoplasm has a tendency when activated to form 

 asters spontaneously (Morgan, Wilson). When the 

 sperm nucleus is present all such activity centers 

 around it and is usually inhibited elsewhere. 



