418 ANN BISHOP 



the division of the body showed between the old and new 

 peristomes. In such cases division would be unequal ; one 

 daughter would be two-thirds the length of the other. On no 

 occasion have I seen any such inequality in size between the 

 two daughters. 



In the individuals of the small variety about to undergo 

 fission the old peristome lies in the anterior third and the 

 new peristome forms in the middle to posterior third of the body. 



Animals about to divide are always much longer than the 

 average-sized individual of the same culture. This length is 

 reached by a gradual process of growth, principally in the 

 region behind the mouth. 



The first indication of coming fission is cytoplasmic, not 

 nuclear. It consists of the formation of the peristomial mem- 

 branellae of the new cytostome. In the major variety the 

 anterior end of the new membranellae is almost level with the 

 posterior curve of the old. The first indication of the formation 

 of these daughter membranellae is a slight ridge in the posterior 

 half of the body, running parallel with the rows of the body 

 cilia. This ridge gradually becomes more pronounced, and 

 along it the new membranellae are formed. At first these are 

 very small but rapidly grow larger. The immature mem- 

 branellae are much shorter in proportion to their width than 

 are the mature ones. This gives them a somewhat leaf -like 

 appearance. The movements of these developing mem- 

 branellae, almost until the separation of the two daughters, 

 are very irregular, many of the individual membranellae moving 

 in different directions to their neighbours, which gives the 

 whole a ragged appearance. This lack of co-ordination in 

 movement is very noticeable when compared with the steady, 

 undulating motion of the mature membranellae. 



The formation of the membranellae is usually well advanced 

 before the nucleus shows any intimation of approaching fission. 

 The times given above w^ere taken from the beginning of the 

 nuclear changes. The somewhat zigzag form of the meganucleus 

 becomes straightened, the lobation is gradually lost, and its 

 shape becomes vermiform. Stained preparations of this stage 



