296 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



embryo and of the yolk sac, and that the velocity of this re- 

 action is not only determined by the mass and character of 

 the yolk in the egg, but also by the mass arid character of the 

 sperm nucleus which enters the egg. Whether or not this 

 latter material consists of enzymes, as has been suggested, 

 need not be considered in this place. 



From this it is also clear why the rate of development in 

 the two hybrids is not identical although the constitution of 

 the nuclei is identical in the two hybrids. The mass and 

 probably the quality of the yolk material is different in the two 

 eggs and hence the velocity of development of the embryos are 

 different. 



In many and especially heterogeneous hybrids still another 

 factor is to be considered, namely, that the foreign spermato- 

 zoon makes the egg sickly and that as a consequence the later 

 development of the embryo is often retarded in comparison 

 with the pure breed. 



This is most strikingly the case in the cross between the 

 sea-urchin and the starfish. As I pointed out long ago the 

 larvae die mostly in the gastrula stage, and possibly one egg in 

 a million reaches the pluteus stage. The development of the 

 pluteus is in such cases always retarded. 



We find such a retardation not only in the case of hetero- 

 geneous hybridization, but occasionally also in the case of 

 crosses between closely related forms. While the hybrid 

 purpuratus ? and franciscanus 6 is vigorous, the hybrid fran- 

 ciscanus ? and purpuratus $ is sickly and reaches the pluteus 

 stage only rarely and slowly. 



3. There are other facts which indicate that in heterogeneous 

 hybrids the spermatozoon has merely a developmental but not 

 a hereditary effect. Moenkhaus found that the eggs of bony 

 fishes can easily be impregnated with foreign sperm, but that 

 they do not develop very far. Thus he states that the hybrids 

 between Menidia and Fundulus heteroclitus "never go beyond 



