xv] SUMMARY 255 



that in the interaction that takes place between nucleus and 

 cytoplasm, each side is more or less modified by the other. 

 If chromosomes of Strongylocentrotus are introduced into 

 Sphaerechinus cytoplasm by fertilising Sphaerechinus eggs 

 by Strongylocentrotus sperm, it can hardly be doubted that 

 the cytoplasm of the larva is modified by the paternal 

 chromosomes, and it is possible, though less easily demon- 

 strable, that the chromosomes of paternal origin also undergo 

 changes, and that if such a hybrid could be crossed back 

 with pure Strongylocentrotus, no larvae with quite unmodified 

 Strongylocentrotus characters would be produced. Such an 

 experiment is probably impossible to perform, for in cases 

 in which the forms crossed differ in only one or few charac- 

 ters, their general constitution is so alike that no modifica- 

 tion of the chromosomes by the cytoplasm is likely to 

 appear. When, however, the forms crossed are sufficiently 

 unlike to make such modification probable, even if the 

 hybrids were fertile t the segregating characters would be so 

 numerous that complete analysis of them would be impos- 

 sible. But if it be true that the factors for Mendelian 

 characters are borne by chromosomes, these factors can only 

 cause the characters to appear by producing modifications 

 in the cytoplasm, and if the chromosomes modify the cyto- 

 plasm, it seems natural to assume that the relation is a 

 reciprocal one, and that the cytoplasm may be not without 

 effect on the chromosomes. That the effect may be rela- 

 tively small is indicated by the persistence of chromosomes 

 of the paternal type through the whole life-history of certain 

 hybrids (for example the moths Biston hirtarius and Nyssia 

 zonaria referred to in Chapter VI), but because the modifi- 

 cation is not large or conspicuous it is not necessarily non- 

 existent. A wide field for research remains open in the inves- 

 tigation of the means and results of this interaction between 

 the nuclear and cytoplasmic constituents of the cell. 



