316 A. FRANKLIN SHULL 



not occur chiefly in periods of depression. Sexual females occur 

 most abundantly in times of rapid growth, rapid development, 

 and large families. What phenomena stand in the relation of 

 cause and effect at these times is uncertain, but the heightened 

 speed of metabolism is usually recognizable before the abundance 

 of sexual females exists. Furthermore, those lines which under a 

 given set of conditions are producing the largest proportions of 

 sexual females are, under like external conditions, the most 

 vigorous lines. This has been the experience, I believe, of all 

 who have worked extensively with Hydatina. 



If depression is not, in parthenogenetic animals, universally 

 associated with the initiation of sexual reproduction, great risk 

 attaches to the assignment of causal significance to other phe- 

 nomena (nuclear enlargement, for example) which accompany 

 depression. The question whether the nuclear changes are 

 merely accompaniments of, or accidentally associated with, re- 

 productive changes, or possibly effects of the agents that also 

 cause changes in reproduction, is comparable to the question 

 already often raised concerning the validity of the Kernplasma- 

 relation theory in other fields. Although Minot ('08) plainly 

 regards a decrease of the nucleoplasma ratio as the cause, or at 

 least a cause, of senescence, and though Hertwig ('08) expressly 

 states that changes of K/P cause or postpone cell division, other 

 authors are either non-commital or regard the nucleoplasma ratio 

 as incidental or even as an effect of the phenomena which it is 

 supposed to govern. Child ('15), for example, considers the rela- 

 tive size of the nucleus incidental as regards senescence, and 

 ConkUn ('12, '13) concludes that so far as the relation of nuclear 

 size to cell division is concerned, nuclear size is an effect instead of 

 a cause. The imphcation of Issakowitsch ('07), therefore, that 

 the supposed relation of nuclear changes to the mode of reproduc- 

 tion in parthenogenetic animals might be merely incidental, and 

 the expressly stated views of Strohl ('08) and Woltereck ('11) 

 that volumetric changes of the nucleus can have nothing to do 

 with the alternation of parthenogenesis and sexual reproduction, 

 find a counterpart in the ideas of workers in other fields. In 

 view of the results with Hydatina described above, it may be 



