NUCLEAR VOLUME AND LIFE-CYCLE OF HYDATINA 289 



continued vegetative reproduction until such a size is reached as 

 incapacitates the cell for further activity by preventing the usual 

 metabolic processes. In this condition the cell remains until it 

 either, 1) dies or, 2) conjugates or, 3) reorganizes itself by eliminat- 

 ing some of the nuclear matter into the cytoplasm. By either 

 of the last two processes the 'normal' relative volume of the 

 nucleus to cytosome ('Kernplasmanorm,' Hertwig, '08, p. 9) is 

 restored, and the cell rendered again capable of a series of vegeta- 

 tive divisions. 



Extension of this theory to other cyclical phenomena was no 

 doubt hastened by the discovery that the two sexes of certain 

 animals, or at least their germ cells, are distinguished by different 

 nucleoplasma ratios. Hertwig ('05) conceived that any increase 

 of the quotient K/P would lead to the production of males, de- 

 crease of that quotient favoring females. Inasmuch as the alter- 

 nation of parthenogenesis and sexual reproduction was then re- 

 garded, and by many biologists is still regarded, as a phenomenon 

 of sex determination, those who were working with such alternat- 

 ing cycles were quick to suggest that volumetric changes of the 

 nucleus are responsible for the change from one mode of reproduc- 

 tion to the other. On the basis of experiments performed by 

 Issakowitsch on Cladocera, Hertwig ('05) concluded that the 

 change from the parthenogenetic to the sexual mode of reproduc- 

 tion is a direct result of an increase of the Kernplasmarelation; 

 that is, of an increase of nuclear volume relative to cytosomal 

 volume. This view was adopted by Issakowitsch ('07). It does 

 not appear that anj^ measurements of nuclear and cytosomal 

 volumes in the daphnids were made by Issakowitsch. The 

 evidence obtained from the daphnids was indirect. Long-con- 

 tinued parthenogenesis led to a 'physiological degeneration,' one 

 sign of which was the disintegration of eggs in the brood chamber 

 at the end of such a series. Since in Protozoa these periods of 

 depression ('physiological degeneration') were preceded or accom- 

 panied by an enlargement of the nucleus, it was assumed by 

 Hertwig and Issakowitsch that a similar increase of the nucleus 

 probably occurred also in the daphnians. Furthermore, the ef- 

 fect of temperature upon Protozoa and the daphnians, while not 



