PROTOZOA 



35 



pression, in which the nucleus^ is overgrown, the body stunted, divi- 

 sion retarded, and the various organs and functions increasingly de- 

 generate, until finally digestion ceases and the organisms die. From 

 this condition conjugation will recover a culture which is not too far 

 gone. It was held that depression was the senility of the organism — 

 ultimately of the same nature as that which in the Metazoa destroys 

 the parent body, while the gametes, after syngamy, continue the 

 existence of the species — and the conclusion was drawn that in both 

 cases the effect of the union of nuclei was rejuvenation. Now, how- 

 ever, it is known that depression is a disease, which by more natural 

 methods of culture can be avoided without conjugation. It is true that 

 in cultures of ciliates there has been observed a periodical waxing 



<^ 



Cg5>>c2g><^ 



Fig. 28. A diagram of the nuclear changes in Paramecium aurelia during 

 endomixis. From Robertson, after Jennings. The white circles represent 

 degenerating nuclei. Fissions take place between D and E, and between H 

 and I. 



and waning of vitality of which the low points in some cases coincide 

 with conjugation ; but in other cases there occurs at these points not 

 conjugation but a process known as endomixis, which closely resembles 

 the procedure in conjugation, but takes place in solitary individuals 

 and does not involve syngamy. In this process (Fig. 28) the mega- 

 nucleus is destroyed and replaced by one of the products of the 

 division of the surviving micronucleus, as in conjugation. It would 

 appear from these facts that the invigorating effects of conjugation 

 are due not to the true syngamy (union of nuclei) but to the accom- 

 panying replacement of the meganucleus, which probably has become 

 effete (see p. 27). If, as has been suggested (p. 26), those protozoa 



^ In ciliophora the meganucleus. 



