CHROMATIN — DEVELOPMENT AND HEREDITY 239 



lus on the spermatazoon of Ctenolabrus in the reciprocal cross 

 in that it emphasizes the active part taken by the egg in its re- 

 action with the spermatozoon. 



In heterogeneous crosses, then, the spermatozoon exercises no 

 influence upon the rate of cleavage of the egg. This is not true 

 of homogeneous crosses, as both Newman and the Hertwigs 

 have shown. In the crosses Fundulus majalis 9 X Fundulus 

 heteroclitus cf and Gobius capito 9 X Gobius jozo cf develop- 

 ment is more rapid than in either of the egg parents. The con- 

 clusion is that here the role of the sperm is a positive one. These 

 facts suggest that the spermatozoon plays a more passive part 

 in heterogeneous fertilization than when the species crossed are 

 more closely related. They also indicate that it is the reaction 

 between the egg cytoplasm and the spermatozoon chromosomes 

 which causes the division of the latter and that this reaction is 

 to be regarded as distinct from the cooperation between the chro- 

 matin elements of the conjugating nuclei which determines the 

 hereditary character of the 'arva. 



The fact that polyspermy is the rule in heterogeneous teleost 

 hybridization, rather than the exception, lends weight to the 

 argument that the ease with which the eggs of any species of 

 fish can be fertilized by the spermatozoa of any other species is 

 due to the general similarity in the physical organization of the 

 teleostean germ cells and depends upon physical or chemical 

 qualities quite distinct from the hereditary constitution of the 

 germ substance and independent of it. In other words, the ease 

 with which cross fertilization is carried out is not dependent 

 upon the degree of relationship of the species crossed. The 

 term fertilization as used here is meant to include both the 

 process of insemination and the activities resulting in the union 

 of the egg and sperm nuclei and their cooperation in mitosis. 

 Polyspermic eggs afford some evidence that the participation of 

 the paternal chromatin in mitosis in the teleost egg is due to 

 mechanical or chemical factors of a general nature. Figure 34 

 shows a section through an egg which divided into three cells 

 instead of two at the first cleavage division. There is no direct 

 proof that this is due to polyspermy, but the combined evidence 



