246 Mechanism of Mendelian Heredity 



in certain cases, for instance why the influence of certain 

 hereditary factors for pigmentation should depend 

 upon temperature as E. Baur observed. 



The view, according to which the determiners in the 

 chromosomes only tend to give special characters to 

 the embryo or to the adult while the cytoplasm of 

 the egg may be considered the real embryo, receives 

 some support from the fact that the first development 

 of the egg is purely maternal, even if the egg nucleus 

 has been replaced by sperm of a different species. 

 If an egg of a sea urchin be cut into two pieces, one 

 with and one without a nucleus, and the enucleated 

 piece be fertilized with the sperm of a different species of 

 sea urchin, the blastula and gastrula stages are purely 

 maternal and only the skeleton of the pluteus stage 

 begins to betray the influence of the foreign sperm 

 inasmuch as this skeleton is purely paternal, according 

 to Boveri. In all experiments on hybridization it has 

 been found that the rate of cell division of the egg is a 

 purely maternal character. Thus when fish eggs of a 

 species, in which the rate of first segmentation of the 

 egg is about eight hours, are fertilized with sperm of a 

 species for which the same process requires about 

 thirty minutes or less at the same temperature, the 

 rate of segmentation is again about eight hours. There 

 is then no chromosome influence noticeable in the early 

 development. 



When two forms of sea urchins, Strongylocentrotus 



