224 V. A. DOGIEL 



nucleus ; while the larger continues to swell and becomes the 

 new macronucleus (PI. 17, fig. 5, Ma). 



Several particularly successful preparations help us to 

 elucidate the further fate of the micro-exconjugants. They 

 complete their reorganization and return to the type of the 

 neuter individuals. Specimens with the nuclear apparatus 

 not fully reconstituted show the rudiments of a new and very 

 thin skeletal plate, which is finely alveolar — its alveoli corre- 

 sponding to those ultimately constituting the fully formed 

 plate. At the same time the fibres of the pharyngeal muscular 

 mantle apply themselves to the inner surface of the plate, and 

 in so doing form the pharyngeal diaphragm. Further modifica- 

 tion produces tiny neuter specimens, of the same length as the 

 microconjugants but with fully developed skeletal plates. It 

 might be objected that the stages described can be interpreted 

 in an inverse sense, i.e. that the tiny neuters by losing their 

 skeletal plates may become microconjugants. But there can 

 be no such alternative, for the formation of microconjugants 

 takes place in quite a different way. They arise as a result 

 of an unequal fission of neuter individuals. 0. janus 

 possesses two different kinds of fission. One of them, the 

 ordinary one, leads to the formation of neuters. The other 

 one, which may be called progamic , results in the formation 

 of two preconjugants which differ in size and other morpho- 

 logical characters. The beginning of the fission is in both 

 cases manifested by the elongation of the posterior half of the 

 body and of the macronucleus as well, while the micronucleus 

 assumes the shape of a short spindle. At the same time the 

 first rudiments of the new adoral and dorsal zones of mem- 

 branellae (PI. 17, fig. 6, D^-g) appear under the cuticle. 



In cases of ordinary fission (PI. 17, fig. 6) the phase just 

 described clearly indicates the initial stages of the building up 

 of a new skeletal plate in the posterior individual (PI. 17, fig. 6, 

 skp2)' Further phases lead to the complete formation of the 

 posterior individual, with fully developed skeleton and all the 

 characters of the neuters. As a result of fission we have two 

 neuters with long terminal spines. During the w^hole process 



