294 A. FRANKLIN SHULL 



therefore, show a different Kernplasmarelation from those reared 

 in spring water. 



Hydatina has also the advantage of being free from certain 

 objections which pertain to other material. First, the cells do 

 not divide after the adult condition is reached. Martini ('12) 

 showed that the number of cells was (usually) the same in all 

 individuals. Since this is as true of young adults as of old ones, 

 it follows that there are no cell divisions in the adult (except, of 

 course, the maturation division of the oocyte). Although it has 

 since been pointed out (Shull, '18) that the cell number is not as 

 constant as Martini supposed, there is nothing in the new dis- 

 coveries to indicate that cells divide in the adult. The absence 

 of cell division eliminates one source of error in the measurement 

 of the Kernplasmarelation in other tissues than the oocytes, since 

 there is no danger of measuring cells just prior to division, at 

 which time the nuclei become rapidly larger ('divisional growth' 

 of Hertwig). 



Second, there is no chromidial apparatus in the cytoplasm 

 which might interfere with an accurate determination of nuclear 

 volume — a difficulty which Hertwig found in certain Protozoa. 

 The yolk gland of Hydatina occasionally contains deeply staining 

 masses which are probably not related to chromatin, but no 

 such specimens were used in this work. 



Third, differences in environment need not introduce any dis- 

 crepancies in K/P, the Kernplasmarelation, since the animals 

 are small and their life short. Except in those experiments in 

 which differences of environment were intentional, the conditions 

 under which animals to be compared were reared were alike and 

 highly uniform. 



Even the foregoing advantages of Hydatina for the study of 

 the Kernplasmarelation might not have led me into so laborious 

 an investigation, but for the further circumstance that a dif- 

 ference in the quantity of chromatin between the male-producing 

 and female-producing individuals might thereby be detected. 

 From a cytological study no difference in chromosome number 

 had been detected (Shull, '21), but it was suggested to me by 

 several biologists that the quantity of chromatin might neverthe- 



