FERTILIZATION 183 



some. The aster doubtless is formed out of the egg cytoplasm, 

 by the influence of the centrosome or centrosome-forming 

 substance of the spermatozoon; and it is quite possible that the 

 centrosome itself may be similarly formed from the cytoplasm 

 through the action of some chemical substance introduced by 

 the sperm. For it is known that the cytoplasm of the egg 

 does possess the property of forming asters with typical centro- 

 somes, under the influence of appropriate "artificial" stimulus 

 (Yatsu). And recently Lillie has shown, in Nereis, where the 

 middle piece does not enter the egg at all, that the centrosome 

 forms in association with the sperm nucleus, even when only a 

 small portion of this is allowed to enter the egg. This indicates 

 that the centrosome, as well as the aster, results from the 

 redisposition of substances of the egg cytoplasm following the 

 entrance of the spermatozoon. A conclusion as to whether or 

 not the law of genetic continuity applies to the centrosome in 

 fertilization is less important than recognition of the uniformity 

 of its chemical and physical actions, in either case. And 

 although the centrosome as an organized body may disappear 

 in the spermatozoon, this still contains kinoplasmic substance 

 of an equivalent function. Here, as elsewhere, the essential con- 

 tinuity may be chemical rather than morphological, but for 

 that reason it is not to be regarded as any less actual or 



portant. 



While the spermatic structures have been thus active, the 

 egg nucleus has been completing its maturation, at the con- 

 clusion of which the egg centrosomes and asters have disap- 

 peared (Figs. 94, 95). The egg nucleus is left near the surface 

 of the animal pole, either near the sperm nucleus or at some 

 distance from it. There are now present in the ovum all of 

 the chief elements which are to take part in the essentials of 

 fertilization and development. These are (1) the egg nucleus 



o 



with its ^ chromosomes, either distinct or formed into a 

 characteristic nuclear reticulum, and with or without a nuclear 



o 



membrane; (2) the sperm nucleus, also known to contain -= 



