164 DOXXELL B. YOUNG. 



fully studied under the microscope and any in which the plane of 

 cutting was such that either part of the nuclear complex might 

 have been injured were discarded. 



Seventeen animals were selected and cut in a median transverse 

 plane, thus separating the two halves of the nuclear complex. 

 The parts were kept under observation and the process of re- 

 generation studied. In eight or ten hours each fragment had 

 developed its missing structures. At the end of thirty-six hours 

 most of the regenerated individuals had divided. Some of those 

 which had already divided as well as some which had not yet 

 started division were stained. In every case the nuclear com- 

 plex was complete, that is, there were two macro- and two 

 micronuclei. 



Ten individuals were next cut in the same plane and in sixteen 

 hours the pieces were stained. Regeneration was complete in 

 both nuclear and cytoplasmic structures. No division had taken 

 place. There was no indication of the method of nuclear 

 reorganization. 



Eight other individuals were stained about six hours after 

 cutting. At this time the cytoplasmic regeneration was prac- 

 tically complete. The body form was normal and the number 

 and arrangement of cirri correct, although some of the cirri 

 were not full size. In each animal studied, the micronucleus 

 was in the process of mitotic division and the macronucleus had 

 begun to separate into two parts. 



Eight individuals were stained three to five hours after cutting 

 in the same plane as before. The cytoplasmic structures had 

 begun to regenerate but the process was far from complete. 

 The micronucleus showed various stages of division up to about 

 a metaphase condition. The macronucleus in some cases had 

 not begun to divide while in other cases it showed a slight con- 

 striction. 



This study seems to indicate that in the race of Stylonychia 

 mytilus used in this series of experiments the nuclear complex 

 is constant and the whole complex is necessary for the normal 

 life of the animal. Any cut which separates the two halves of 

 the nuclear complex results in the regeneration of the normal 

 complex by the amitotic division of the macronucleus and the 



