NUCLEOPLASMIC RELATIONS IN ARCELLA 89 



this also holds true for other species of Arcella, but evidence was 

 obtained that indicates that no difficulty would be experienced 

 in securing similar results with A. polypora. In family ap. 34 

 two lines were noted which contained specimens with the same 

 number of nuclei (7), but whose mean diameter showed a differ- 

 ence of 3.28 units of 4.3 m each. It was not possible to carry on 

 these lines, and so only a few specimens were reared (thirty- 

 seven in all), but even this small number is significant in the light 

 of the data we already possess in other species of Protozoa. 



7. Chromidia 



Arcella is one of the first organisms in which extra-nuclear 

 granules, now known as chromidia, were discovered. A band of 

 these granules w^as described by R. Hertwig in 1887, and the 

 same investigator twelve years later (Hertwig, '99) recorded the 

 formation of secondary nuclei from this 'extra-nuclear chromatin 

 net.' These nuclei, according to Elpatiewsky ('07), become the 

 centers of amebulae of two sizes, which are macrogametes and 

 microgametes and which conjugate in pairs. The conjugation 

 of the entire chromidial nets of pairs of Arcellas was reported the 

 following year by Swarczewsky ('08) . Chromidia have been de- 

 scribed in many groups of Protozoa and seem to play an impor- 

 tant part in their life cycle. For this reason it seems worth while 

 to mention the apparent lack of influence of this extra-nuclear 

 chromatin during the bisection experiments performed on Arcella 

 dentata. These experiments demonstrate that when from 

 one-tenth to three-fourths of the entire chromidial net is removed, 

 the descendants of the part that remains invariably attain the 

 normal condition of the line. When a large Arcella is bisected 

 and both nuclei are present in one-half, the other half without 

 nuclei always dies within a few days without any visible attempt 

 on the part of the chromidia to form new nuclei. This is true 

 even when only the nuclei are removed and all the cytoplasm is 

 left in the shell. The evidence justifies the conclusion that the 

 chromidia play no role in vegetative reproduction and at this 

 time have no influence upon the size of the organism nor upon 



