Chromatin as '^' Hereditary Substance^'' 345 



the male chromosomes, should development ensue under the 

 impetus of both artificial parthenogenesis and artificial fer- 

 tilization. The animals used were species of two genera of 

 sea-urchin Spliaer echinus and Strongylocentrotus, tlie eggs 

 being from the first and the sperm from the second. A few 

 minutes after the impetus to parthenogenetic development 

 had been given by treating them with a weak solution of 

 valerianic acid, the eggs were removed to normal sea water 

 and mingled with the sperm of Strongylocentrotus. The 

 sperm fertilized some of the eggs, but since the nuclear 

 changes of the male nucleus within the egg always lagged a 

 little behind the changes of the female nucleus, it happened in 

 some instances that the male nuclei passed into one only of 

 the two first blastomeres, the result being that in the em- 

 bryos in the two-cell stage, one cell contained only a female 

 nucleus, while the other contained both a female and a male 

 nucleus, which in some cases fused in the usual fashion mak- 

 ing a larger nucleus than that of the other cell. This could 

 be made out by direct observation. It was further obsei*\ed 

 that in batches of eggs where a two-cell stage of this sort 

 occurred, the resulting larva possessed a typical hybrid 

 skeleton on one side, and a skeleton typical in several respects 

 of Sphaerechinus on the other. 



"The female skeletal side," says Herbst, "corresponds to 

 the small nucleus designated as left, and tlie hybrid to tlie 

 large one designated as right." Although no details are 

 given as to the exact relation of the two kinds of nuclei to 

 the skeletal elements presenting characters from two diverse 

 species, the inference is hardly to be escaped that the rela- 

 tion is one of actual causal dependence. But Herbst's atti- 

 tude of caution and restraint must not be ignored, as it 

 seems to have been by some of the supporters of the chro- 

 mosome theory whose enthusiasm seems to be too strong for 

 their judgment. He is careful to point out that he found 

 no lai'va in which the part of the skeleton presenting mater- 



