28 ROBERT W. HEGNER 



d. Experiments on members of family 58 



1. Line 153. The progenitor of this Hne was a member of 

 the high Hne of family 58 which was being used for selection 

 w^ork when this series of experiments was begun (January 10, 

 1918). The specimen operated upon had 12 spines and was 

 28 units in diameter (fig. 25), and the hne from which it was 

 taken had at the same time a mean spine number of 11.48 and 

 a mean diameter of 27.26 units. The pedigree shown in figure 

 26 gives part of the history of this Hne. The half of the parent 



Fig. 24 Arcella dentata. Outline of the first offspring of the progenitor of 

 family 152. The cross line indicates where the specimen was bisected. X 207. 



Fig. 25 Arcella dentata. Outline of a specimen of family 58 which served as 

 the progenitor of line 153. The cross line shows where the specimen was bisected. 

 X 207. 



Fig. 27 Arcella dentata. Outline of the progenitor of line 155, showing where 

 it was cut into four pieces. X 207. 



labeled 153a threw an empty shell immediately and the rest of 

 its offspring and other descendants were all binucleate. The 

 half labeled 153b doubled after producing three offspring. 

 Nuclear doubling throughout this entire line occurred earher 

 than in the lines already described, but there is no apparent 

 reason for this behavior and it is probably of no significance. 

 The data are presented in compact form in table 7. The mean 

 spine number and mean diameter of the binucleates is lower 

 than that of the line from which they were descended because 

 the small specimens that were produced immediately after 

 nuclear doubling are included. Many of these had onlj" 8 or 



