Phenocopy and Norm of Reaction 255 



cent regenerate. But when the nucleated part of the species med is 

 combined with the tip of acic a cap is regenerated. The cytoplasm of 

 the regenerating piece has not been changed, but the med base 

 supplies the necessary formative substances. 



We mentioned, earlier, Boveri's famous proof that a chromosomal 

 behavior, chromatin diminution, is controlled by cytoplasmic sub- 

 stances. This can be shown in Acetabularia also. Before reproduction 

 the single nucleus divides into many, which migrate into the cap to 

 form the gametes. This happens only when the cap has reached a 

 definite size. If the cap is cut off in time, the nucleus does not divide, 

 but first controls the production of a new cap; thus gamete formation 

 could be held up for two years. Vice versa, if an immature base re- 

 ceives the transplant of a mature cap, the nucleus begins at once to 

 divide. The substances involved in this induction can be shown to be 

 species-specific. 



These experiments provide an excellent model within a single 

 cell, demonstrating, in a general way, how genie material in the 

 nucleus is connected with specific determination in differentiation. 



PHENOCOPY AND NORM 

 OF REACTION 



A. NORM OF REACTION 



Another group of facts of a generalized nature should be kept in 

 mind at the outset of a study of genie action. In the early years of 

 Mendelism, Woltereck defined the genotype as a "norm of reaction." 

 What he meant was that the genie action does not control a definite 

 phenotypic effect but an effect which is the resultant of the genie ac- 

 tion and that of external and internal environment. A certain mutant 

 does not control the deposition of black pigment, but has this action. 



