420 ANN BISHOP 



forwards with the meganucleus, but that they become much 

 swollen and stam very lightly (PI. 22, fig. 4, M.N.). At this 

 stage, when the meganucleus lies in the anterior end, the micro- 

 nuclei are almost three times their normal size. They are 

 surrounded by a distinct halo. Some are close to the mega- 

 nucleus, but others are a slight distance away from it. 



The meganucleus moves backwards until it comes to lie in 

 the middle of the animal's length, at the same time contracting 

 still further (Text-fig. 7). The movement backwards to the 

 middle of the body is fairly rapid. Here the meganucleus 

 remains for a relatively much longer period. In appearance 

 it is a roughly oval, dense structure. Its form is not rigid 

 but slowly changes, showing protuberances at the side which 

 gradually disappear and reappear at other places. 



In many sections of animals with the meganucleus fully 

 contracted in the centre of the body it has not been possible 

 to find any micronuclei. In one or two specimens large, 

 pale-staining micronuclei have been seen ; these evidently 

 have not yet undergone division. In other cases small, intensely 

 staining micronuclei have been found near the meganucleus. 

 It seems quite probable that these are the products of division, 

 and I hope to say more about them in a further paper. In all 

 further stages of division in Spirostomum ambiguum 

 the micronuclei are small and stain normally. 



From the fact that, when the micronuclei of conjugants 

 are about to divide, they become swollen and similar in appear- 

 ance to the ones described above, it is to be concluded that 

 these micronuclei divide some time during the migration back- 

 wards of the anteriorly contracted meganucleus, or soon after 

 it takes up its central position. Unfortunately I have never 

 actually seen any division taking place in any preparations, 

 and conclude therefore that the division must take place very 

 rapidly. A similar difficulty was experienced by Johnson (13) 

 whilst working upon the division of Stentor. 



During the time when the meganucleus is fully contracted 

 in the centre of the body, a slight dilation becomes visible 

 in the feeding canal of the contractile vacuole, on a level 



