NUCLEOPLASMIC RELATIONS IN ARCELLA 85 



3. Chro77iatin-cytoplasmic relations in Arcella 



The morphology of the nuclei of Arcella makes it possible 

 to study the mass relations between the chromatin and the 

 cytoplasm, since the former occurs as a nearly spherical body 

 suspended in the nuclear sap. This 'chromatin mass' may 

 contain plastin or some other substance besides chromatin, 

 but the amount of chromatin probablj^ varies as the size of the 

 mass. It was very easily determined in Arcella dentata that 

 in two different lines that differed in size the chromatin masses 

 within the nuclei of the larger were greater than those within the 

 nuclei of the smaller. A clear relation between chromatin mass 

 and cytoplasmic mass was thus established. 



A similar conclusion was reached with respect to different 

 lines of Arcella polypora. In this species there was found to be 

 a constant difTerence in size between specimens with the same 

 number of nuclei belonging to different lines. When, how- 

 ever, the masses of these nuclei were computed and the total 

 volume of chromatin was determined, the results showed that 

 here also there is a definite mass relation between chromatin 

 and cytoplasm. The number of nuclei might be different in 

 specimens belonging to different lines, but the size is the same 

 whenever the total volume of chromatin is the same. 



In one case the data seem to favor Minot's theory that senes- 

 cence and death are due to an excess in the amount of cyto- 

 plasm. From a line of Arcella dentata containing very small 

 specimens there was derived a line consisting of enormously 

 large ones. These, after several generations, failed to repro- 

 duce, and on examination, their nuclei were found to be no 

 larger than those of small members of the parent line. Their 

 subsequent death may have been due to the failure of the organ- 

 isms to regain the normal racial relations between chromatin 

 and cytoplasm. 



4. Changes in nuclear number and the formation of empty shells 



This subject involves several interesting questions that can- 

 not be answered with any degree of certainty. Some of these 

 questions are as follows: 



