328 SCIENCE PROGRESS. 



in an intervening space (fig. 17). At the same time 

 the filaments connecting the daughter nuclei of each 

 animal disappear, and each of the two approximated nuclei 

 passes from its parental animal into the other conjugate. 

 In this way there are still two micro-nuclei in each con- 

 jugate, but one of them is indigenous to the animal in 

 which it lies, while the other is derived from the opposite 

 conjugating individual. The two micro-nuclei in either 

 animal at first lie closely side by side (fig. 18), but eventu- 

 ally their investing membranes coalesce, and they fuse 

 gradually together (figs. 18, 19). The process of conjuga- 

 tion is now complete, and there is one micro-nucleus in each 

 conjugate, as represented in fig. 19. 



Thus the essential process in the long metamorphosis 

 is a transference of one micro-nucleus from each indi- 

 vidual to the other, and the fusion of each of these trans- 

 ferred corpuscles with that which remains in its parental 

 cell. The compound micro-nuclei thus produced are 

 obviously similar to the nuclei of the fertilised ova in the 

 higher animals and plants, and it is evident that the 

 fertilisation process of the ciliates is in all respects identical 

 with that obtaining among metaplastid cells. Thus it follows 

 that the conjugation period among the Infusoria can be used 

 as a point for comparison between the life cycle of these 

 animals and the development of metaplastid cells ; and it is 

 possible further to compare the succeeding events in the 

 Infusoria with the early developmental phenomena of the 

 higher forms. 



After the separation of the conjugates, the fertilised 

 micro-nuclei in each Paramcecium divide rapidly three 

 times (figs. 20, 21, 22, 23), in such a way that of the eight 

 residual nuclei four lie at one end of the animal and four at 

 the other ; and these nuclei, remaining some time connected 

 with one another in pairs, give to the body of the animal the 

 very curious appearance represented in fig. 24. 



After a time the connections between these two groups 

 disappear, and their respective characters — if not from the 

 first inherently different — become subsequently changed. 

 Those at one end grow large and macro-nuclear, while the 



