NUCLEOPLASMIC RELATIONS IN ARCELLA 65 



formed at the time of nuclear doubling, as was found to be the 

 case in A. dentata. The difference in size between the uninu- 

 cleate and binucleate specimens of A. discoides is, however, 

 very similar to that recorded above for A. dentata. 



d. Summary of results of ohservations and experiments on Arcella 



discoides 



1. Families reared from wild specimens of various sizes prove 

 the existence of different pure lines with regard to diameter. 



2. Variations in diameter within a family are less than those 

 in A. polypora and more nearly like those in A. dentata. This 

 is due to the constancv of the nuclear number. 



A B 



Fig. 43 Arcella vulgaris. Sketches showing side views of specimens belong- 

 ing to two different families that differed heritably in the shape of the shell. A 

 is from family ab. 7, and B from family avd. 1. X 207. 



3. Uninucleate pieces of bisected specimens gave rise to a 

 number of small uninucleate descendants, but nuclear doubling 

 occurred in every case, without the intervention of an empty 

 shell, and the mean diameter of the race was regained by 

 members of the second generation. 



4. The difference in diameter between the specimens with 1 

 nucleus and those with 2 nuclei is similar to that recorded for 

 A. dentata and indicates that there is in this species a definite 

 mass relation between nucleus and cytoplasm similar to that 

 described in the latter. 



5. OBSERVATIONS AND EXPERIMENTS ON ARCELLA VULGARIS 



Many specimens of A. vulgaris were found clinging to the 

 duckweed in the fresh-water ponds near Cold Spring Harbor. 

 Eight specimens shaped like that shown in figure 43, A, were 



