SOME CURRENT THEORIES 439 



Muhlmann, it suffers most of all. That accumulation of structural 

 substance and so-called metaplasm in the cells is the result of a 

 gradual starvation is difficult to believe in view of the fact that 

 during actual starvation in the lower animals these substances may 

 disappear to a greater or less extent. And the fact that cell division 

 can be inhibited by starvation is scarcely in agreement with Miihl- 

 mann's assertion that cell division results from starvation of the 

 nucleus. Muhlmann regards all that is commonly called progres- 

 sive development as a regression or involution from the embryonic 

 condition and maintains that the only progress is the reproduction 

 of embryonic cells, but here again we have merely assertion, not 

 evidence. In what way progress is involved in the reproduction 

 of embryonic cells he does not attempt to show. And his assertion 

 that cell division and the cessation of life are both death leaves the 

 idea of death without any physiological significance, for cell division 

 and the cessation of life are certainly two very different processes. 

 In the one an increase in metabolism apparently occurs, while in 

 the other metabolism ceases. 



More than twenty years ago Richard Hertwig ('89) advanced the 

 "opinion, based on studies of certain protozoa, that " depression" and 

 "physiological degeneration" of the cell conditions supposedly 

 more or less closely identical with senescence and natural death 

 are associated with an increase in the size of the nucleus relatively 

 to the cytoplasm, and in later papers ('03, '08) he has attempted to 

 show that the nucleoplasmic relation, i.e., the size ratio of nucleus 

 to cytoplasm, varies and regulates itself within definite limits for 

 each particular kind of cell and that its variation is an index of 

 the functional condition of the cell. This idea has been further 

 developed by some of his students and others, but has also been 

 rather widely criticized, and many investigators have not been 

 able to find the definiteness of relation which Hertwig believes to 

 exist. Conklin ('12), for example, concludes from an extensive 

 study of the nucleoplasmic relation in the development of the 

 mollusk Crepidula, that it is neither a constant nor a self-regulating 

 ratio and not a cause of cell division, as Hertwig believes, but 

 rather a result. As a matter of fact differentiation and senescence 

 in the higher animals are associated in most tissues with an increase 



