260 GENETICS 



Thus while the chromosomes with their invisible 

 genes are the ultimate determiners of heredity, the 

 enveloping cytoplasm that surrounds the nucleus, par- 

 ticularly of the egg-cell, may be the immediate arbiter 

 of the differentiation processes that characterize soma- 

 togenesis. "In short," as Conklin says, "the egg cyto- 

 plasm determines the early development and the sperm 

 and egg nuclei control only the later differentiations. 

 . . . The chromosomes are chiefly concerned in 

 heredity, the cytoplasm in development." 



There is nothing in what has been said of "cyto- 

 plasmic inheritance," however, to conflict with the 

 generalization that the real determiners of heredity 

 are germinal, for it is the genes in the nucleus of the 

 parent germ-cell that gives the character to the egg 

 of the daughter-cell, both to its nucleus and to its cyto- 

 plasm, although the latter in turn influences particu- 

 larly the early stages of somatogenesis. In an ex- 

 cellent criticism of the role of nucleus and cytoplasm 

 as vehicles of heredity, Dunn 1 concludes : "For de- 

 velopment, its mechanism is but grossly known, but we 

 have learned enough of the determinative effect of the 

 nucleus and of the possibilities of interaction betweei) 

 cytoplasm and nucleus to foster a suspicion that one 

 day the governance of the chromosomes over develop- 

 ment will be explained in physical terms." 



7. THE PHYSICAL STAGE-SETTING 



During development the organism is beset on all sides 

 by various external physical factors, which are more 

 *Amer. Nat. vol. LI, 1917, p. 286. 



