10 A. FRANKLIN SHULL. 



The view that vigor depends upon the heterozygosis of the 

 individual seems to me inherently more probable than that it is 

 due to the presence of certain dominant genes. The former view 

 admits of a plausible foundation in cell physiology, and the 

 essence of it may be extended to cases of decrease of vigor in which 

 there is no change in the genotypic constitution, and which are 

 therefore without the pale of either theory. 



Loss OF VIGOR NOT ACCOUNTED FOR BY THE MENDELIAN 



EXPLANATIONS. 



It has been shown in a former paper (Shull, 1912) that par- 

 thenogenetic lines of Hydatina may, and usually do, become less 

 vigorous as parthenogenesis proceeds. This conclusion has been 

 confirmed by Whitney (1912). The same phenomenon, though 

 disputed by Woltereck (1911), has been reported in Cladocera by 

 Papanicolau (1910). Some workers have found that clones of 

 Parameciiim decrease in vigor with long continued fission, though 

 Woodruff (1911) has shown that this phenomenon is not uni- 

 versal. In none of these cases, so far as known, is there any 

 change in the genotypic constitution throughout the line; hence 

 a change from heterozygosis to homozygosis is not responsible 

 for the decrease of vigor. The loss of vigor usually spoken of as 

 senescence likewise occurs without, so far as known, any change 

 in zygotic constitution. 



The physiological explanation which I am about to offer in- 

 cludes the view that heterozygosis determines vigor, and covers 

 also the cases of parthenogenetic lines and clones just mentioned, 

 and perhaps also of senescence. 



A PHYSIOLOGICAL EXPLANATION OF VIGOR. 

 Vigor may be thought of as dependent on the rate of meta- 

 bolism. Lillie (1912), in his studies of fertilization, concludes 

 that the increased metabolism that accompanies the development 

 of the egg is due to an accelerated interaction between nucleus 

 and cytoplasm. The introduction of new nuclear elements into 

 the cytoplasm of the egg, which occurs in cross fertilization, 

 may be supposed to disturb the equilibrium, create a greater 

 reaction between nucleus and cytoplasm, thereby increasing 

 metabolism, and hence vigor. On this view, it is not the fact 



