UROLEPTUS MOBILIS ENGELM. 



463 



nucleus appears as a spherical vesicular space in the center of the 

 ex-conjugant. At forty-eight hours the vesicle is elongate or 

 ellipsoidal, and it continues to enlarge each day until on the 

 fourth day it becomes large enough to make up fully one-third 

 or even one-half of the volume of the cell. Usually, on the fifth 

 day, it condenses into a relatively small macronucleus with 

 dense chromatin contents, and becomes invisible in the living 

 organism (fig. 9). The nucleus is now ready for its first divi- 

 sion, and this, with division of the cell, usually occurs on the 

 sixth or seventh day. 



Reorganization after conjugation thus requires a definite 

 period of time during which there is no reproduction and during 

 which characteristic morphological structures are visible in the 



NOTiNiAL 



y^ Vi 



Fig. 9 Reorganization stages of a normal ex-conjugant from a living indi- 

 vidual on successive days. 



living cell. The latter is an important fact in connection with 

 these cutting experiments. 



Many of the individuals that were isolated after conjugation 

 cutting lived for a longer or shorter time during the reorganiza- 

 tion processes, but died without dividing. Thirty successful 

 operations yielded only eleven individuals which completed the 

 reorganization processes, and four of these died out shortly 

 after the five lines of each series were established, the other 

 seven continued to live, and their histories have been given above. 



In the majority of cases the conjugating pairs were cut in 

 such a manner that the anterior structures of the individual 

 cells, including mouth, peristome, adoral zone of membranelles, 

 and frontal cirri, were removed. In all cases these were re- 

 generated within a few hours, so that the processes of reorganiza- 



