422 ANN BISHOP 



daughter meganuclei just before the cytological separation of 

 the two daughters takes place. 



The newly separated daughters can be distinguished from 

 ordinary individuals by the fact that they are shorter ; and 

 also the meganucleus is vermiform and not moniliform. 

 Further, the cytosome is always at the posterior end, and the 

 I)eristomial membranellae therefore extend practically the 

 whole of the animal's length. Since growth takes place much 

 more rapidly behind the cytostome than in front of it, the 

 cytostome appears to move forwards gradually, so that in 

 animals about to undergo fission it is central in position. 



Lobation of the meganucleus seems to take place at varying 

 times after the separation of the daughters. There seems to 

 be no correlation at all between the number of micronuclei 

 present and the number of lobes formed in the meganucleus. 

 It seems probable, as Collin suggested for the nuclei of Acine- 

 taria, that lobation is governed by the varying tensions of the 

 nuclear membrane. 



C. Conjugation in the Major Variety of 

 Spirostomum ambiguum. 



Stein (28) observed conjugation in Spirostomum ambi- 

 guum on July 28, 1857. Balbiani also observed it in this 

 species. Conjugation, as stated in Part 7, was seen by the 

 present worker in a number of individuals of the major variety 

 of this species during May and June 1922. 



It was noticed that the conjugants were considerably 

 smaller than the ordinary individuals. Stein describes a 

 similar condition in his specimens. It was also noticed that the 

 pairs of conjugants in any one culture showed less variation 

 in size than non-conjugants of the same culture. The greater 

 amount of variation in size in the non-conjugants was probably 

 due largely to the fact that all stages in growth between newly 

 separated daughters and individuals about to divide were 

 present, wherea-s, from the fact that the cytostome was always 

 central in position with regard to the body-length, it was 



