6i8 



SCIENCE PROGRESS 



for a varying number of generations (Fig. 9). These uninu- 

 cleate specimens are smaller than the original parent, but 

 resemble each other in size and spine number. For example, 

 a binucleate specimen with 13 spines and a diameter of 150 

 microns was cut in two, and its uninucleate halves produced 

 209 descendants which had an average spine number of 10-48, 

 and an average diameter of 26-45 microns. These uninucleates, 

 after a few generations, regained the binucleate condition, and 

 as soon as this occurred the diameter and spine number of the 



Four'nonselection periods 3 selection periods 



35 days 23 days 



18 generations 15 gen. 



1325 specimens 722 specimens 



Non- 

 selection 

 periods 

 11 days 

 7 gen. 

 224 spec. 



Fig. 8. 



specimens increased very quickly to that of the original bi- 

 nucleate parent. Operations of this kind were performed 

 many times, and in all 757 binucleate and uninucleate speci- 

 mens were measured and recorded. In every case the results 

 were similar to those just described. The conclusion reached 

 is that each nucleus in a binucleate Arcella is associated with 

 a definite quantity of cytoplasm ; that the quantity that 

 comes within the sphere of influence of a single nucleus may 

 be obtained by determining the mass of cytoplasm within a 

 uninucleate specimen. This mass is doubled when the bi- 

 nucleate condition is regained. 



