REGENERATION IN URONYCHIA 103 



At this time the macronucleus is divided into two nearly equal 

 parts (cf. fig. 3, C). In all cases the micronucleus-holding frag- 

 ment regenerated, while the other did not (fig. 6). 



These thirteen experiments gave no single case of double re- 

 generation of the cut fragments, while in two cases neither frag- 

 ment regenerated. This fact is significant in view of the sup- 

 posedly high growth energy of young cells. 



Fig. 7 Experiment 34. Cell about eight hours after division. A failed to regen- 

 erate but lived for more than 72 hours (see A on right). B regenerated in 24 hours. 



3. Cells cut from 8 to 16 hours after division, i.e. about midway 

 between two division periods 



In fragments of cells cut at times about midway between division 

 periods both the power of regeneration and the rate of regeneration 

 are greater than immediately after division. In the later periods 

 fragments inay regenerate without the presence of a micronucleus. 



Experiments 8, 15, 20, 23, 24, 31, 32, 33, 34, and 38 were based 

 upon cells cut from 8 to 16 hours after division. In nos. 8 and 24 

 one fragment was lost in each case, but the other fragments regen- 

 erated perfectly. In nos. 15, 23, 31, 32, 33, 34, and 38 there 



