68 ROBERT W. HEGNER 



pedigree work carried on with this line, all of the specimens ex- 

 amined possessed 2 nuclei and the mean spine number was 14.25, 

 and the mean diameter was 33.16 units. A distinct change 

 took place during the period of mass culture since the mean 

 spine number of the forty specimens used for chromatin-mass 

 measurements had decreased to 13.32 and the mean diameter of 

 the same specimens had likewise decreased to 31.43 units. 

 Whether this decrease was due to changes in the food conditions 

 or to temperature or to some other environmental factor is not 

 known. The rate of reproduction, on the other hand, increased 

 from one generation in two and one-half days to one generation 

 in slightly less than two days. This was probably due to the 

 higher temperature of summer. 



Line 58 e.g. was a continuation of the high branch of the low 

 line of family 58 described in a previous report (Hegner, '19). 

 It was kept in mass culture under conditions similar to those 

 described for family 150.2ba. The specimens used for chroma- 

 tin-mass measurements represented about generations 129 to 136. 

 When the pedigree culture work was discontinued the mean spine 

 number was 11.38 and the mean diameter was 27.22 units. The 

 mean spine number of the forty specimens whose chromatin mass 

 was measured was 10.29 and the mean diameter was 24.45 units. 

 Here, as in family 150.2ba, there is a similar decrease in both 

 spine number and diametei;, and since both families agree in this 

 respect it seems probable that the environment is responsible 

 for the change. It should be noted that during the selection 

 work already reported simultaneous changes in spine number 

 and diameter took place on several occasions in all the lines 

 at the same time, probably because of some change in the 

 environment. 



The complete history of A. dentata from one generation to the 

 next is not yet fully known, but judging from the small number 

 of specimens that are found in division during the period of day- 

 light, fission probably occurs some time during the night. Popoff 

 ('08) has found that in ciliates there is a gradual growth of 

 nucleus and cytoplasm during the first part of the period between 

 successive fissions and then a very rapid nuclear enlargement 



