ON THE PROTOPLASTID BODY, ETC. 329 



others remain micro-nuclear in character (fig. 25). Further, 

 three out of each of the two groups of four nuclei eventually 

 degenerate and disappear ; and consequently the two 

 remaining nuclei have the character of a micro- and a 

 macro-nucleus respectively. Concomitantly with the above 

 changes the old macro-nucleus has by this time almost 

 entirely degenerated, and consequently the whole nuclear 

 apparatus has returned again into the condition in which we 

 found it, and the animal is ready to fissiparously bipartate 

 once more. 



It is by the rapid multiplication of individuals produced 

 as described above that the familiar and intensely active 

 communities of paramecia arise, and although they may 

 be supplied with endless food and multiply amazingly for 

 a time, it appears, according to Maupas, that their fissi- 

 parous multiplication has its limit. After this has been passed 

 the communities diminish in activity, and the individuals 

 composing them dwindle down in size, and I see no objection 

 to the adoption of Maupas' view that such a community as 

 a whole is undergoing senile degeneration, and that unless 

 some new factor is introduced into the life cycle, it runs 

 the risk of becoming eventually " sans everything". 

 This factor consists in the periodic occurrence of con- 

 jugation in the manner I have just described : and it may take 

 place either among individuals of the same, or, still better, 

 between those of more or less remote communities, resulting 

 in the rejuvenescence of the individuals which undergo 

 it, and the completion of the infusorian life cycle. 



Now in answer to the final question as to the relation 

 between the cyclical evolutions of the protoplastids, and 

 the successional formation of new cells in the development 

 of metaplastid forms, it is well known that development 

 in the latter proceeds, in the majority of cases, from a 

 single fertilised ovum (which, as we have seen, is structur- 

 ally identical with the body of a ciliate or other protoplastid, 

 after conjugation has occurred), and it is carried on by the 

 successive development and arrangement of cells which are 

 also equivalent to the multitude of organisms produced by 

 the successive bipartitions of a protoplastid form. Lastly, both 



