NUCLEOPLASMIC EELATIONS IN ARCELLA 6 



1. INTRODUCTION 



a. The problem 



The investigations described in the following pages were 

 undertaken to determine the relations between nuclear number, 

 chromatin mass, cytoplasmic mass, and shell characteristics in 

 certain species of the genus Arcella. As a rule, genetic research 

 in animals is limited to the study of somatic characteristics alone 

 or to the examination of germ cells that have been killed and pre- 

 pared for microscopic observation. In many cases the germ 

 cells of organisms that have been used in breeding experiments 

 have been studied, but these germ cells have been obtained 

 either from control specimens or from pedigree specimens that 

 have been killed for the purpose. Arcella is pecuharly favorable 

 for investigation because both nuclear and cytoplasmic charac- 

 teristics can easily be seen drawn and measured, at the same time, 

 in the living animal, and their relations can thus be established 

 under the most favorable circumstances. 



h. Advantages of Arcella for nucleocytoplasmic studies 



In a previous paper (Hegner, '19), various characteristics that 

 make Arcella a favorable organism for genetic investigations were 

 pointed out. Among these are: 1) the power of multiplying 

 vegetatively and rapidly; 2) the presence of definite measurable 

 characters that are not modified by growth; 3) the semitranspar- 

 ency of the shell which makes possible the examination of the 

 contents, especially in the recently formed offspring; 4) the abil- 

 ity to withstand severe operations, and, 5) the ease of cultiva- 

 tion and examination. To this list should be added the fact 

 that the nucleus is of the vesicular type with the chromatin, 

 when in the resting stages, clumped into a spherical mass which 

 may easily be drawn and measured. It is thus possible to study 

 the relation between chromatin and cytoplasm, which offers a 

 much more accurate means of comparison than when the entire 

 nucleus with the nuclear sap is involved. 



