218 PHYSICAL BASIS OF HEREDITY 



trotus, was added. The sperm entering the ^gg after its 

 nucleus had started to divide, failed to reach the Qgg 

 nucleus until the latter had divided ( Fig. 100 ) . The sperm 

 nucleus then formed a nucleus of its own, that passed into 

 one only of the daughter cells. This cell got two nuclei. 

 The other cell had only one of the daughter nuclei. Such 

 half -fertilized eggs give rise to larvae that are maternal 

 on one side, and hybrid on the other — or at least larvae 

 of this kind are sometimes found in such cultures (Fig. 

 101), and Herbst believes it is safe to refer them to the 

 half -fertilized eggs. If so, there can be little doubt that 

 the hybrid half owes its peculiarities to the presence of 

 both sets of chromosomes in its cells, while the maternal 

 half owes its peculiarities to its single set of maternal 

 chromosomes. This in itself, however, shows little more 

 than do whole hybrids and whole parthenogenetic eggs 

 themselves, for the demonstration that it is the chromo- 

 somes and not other constituents of the sperm-nucleus 

 that make the difference in the two sides rests on the 

 unproven inference that if other things than the nucleus 

 are involved they would be distributed equally throughout 

 the cytoplasm, but produce no effects. There is no reason 

 to' suppose that they would be so distributed, and no evi- 

 dence that they are. Hence the proof is not cogent, how- 

 ever probable it may seem that only the sperm-nucleus is 

 responsible for those cases where there is a difference 

 in the two sides. 



On the whole, then, while I am inclined to give much 

 weight to this evidence from experimental embryology as 

 very favorable to the hypothesis that the chromosomes 

 carry the hereditary characters, it is the genetic evidence 

 that furnishes convincing evidence in favor of this view. 



