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 and 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 yoik 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 im 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 Q and franciscanus ¢ is vigorous, the hybrid fran- 
ciscanus 2 and purpuratus 2 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 
