382 THE INFUSORIA 



of HOLOTRICHA and HYPOTRICHA respectively, fission is preceded 

 by the formation of a second mouth (Fig. 29, 1), and the growth of a 

 new set of cilia or cirri round and in the neighbourhood of this 

 second mouth, similar in size and arrangement to those in the 

 neighbourhood of the original mouth ; and by the division of the 

 contractile vacuole. In the next phase the micronucleus or micro- 

 nuclei enlarge, then elongate and show the characteristic features 

 of their mitotic division. Next, the meganucleus elongates and 

 becomes constricted in the middle. While these changes in the 

 nuclei are taking place, a constriction of the cortex appears at a 

 point which is approximately half-way down the longitudinal axis 

 of the body. The micronuclei, the meganucleus, and finally the 

 protoplasm of the body, then divide in succession, and the two 

 individuals that are formed separate and swim away. In the 

 process thus described the cortical protoplasm apparently leads the 

 way by the formation of new cilia and a new mouth. On the 

 other hand, the micronuclei have usually formed the mitotic figure 

 before actual constriction of the body is apparent. If the minute 

 structure of the meganucleus be examined in the earlier stages of 

 fission, it may be noticed that it also is not indifferent to the changes 

 going on elsewhere. It is therefore impossible to state with 

 certainty that either the nucleoplasm or the cytoplasm of the 

 organism initiates the process ; in fact, it seems probable in the 

 present state of our knowledge that the impulse to divide affects 

 all parts simultaneously. 



In Spirostomum and Condylostoma the very much elongated 

 moniliform meganucleus is contracted into a short rod-like form 

 before fission occurs. In Stentor, too, fission is preceded by a 

 contraction of the long moniliform meganucleus (Figs. 44 and 29, 2) 

 into a thick spherical lump ; but the meganucleus again elongates 

 and divides transversely into two moniliform bands before the act of 

 fission actually takes place. In Oxytricha (Figs. 16, 17, 18) and 

 some other HYPOTRICHA the meganucleus during the active phases 

 of life is scattered in the form of small granules through the medulla. 

 Before fission takes place these granules collect together and fuse 

 to form a single lump. This consolidated nucleus then divides once 

 or twice, and fission of the whole body follows. 



Fission is not, however, always preceded by the fusion of 

 scattered meganuclei or the contraction of elongated ones. In 

 Opalina the scattered meganuclei appear to be indifferent to the 

 division of the body, and in Opalinopsis (Fig. 19) and Anoplophrya 

 (Fig. 30) the meganucleus divides into fragments, one or more of 

 which become the meganuclei of the daughter individuals. 



In Paramoecium and most of the HOLOTRICHA the transverse 

 fission results in the production of two equal individuals, but in 

 the transverse fission of Stentor and other Heterotricha the anterior 



