I PARAMECIUM 71 



provided each with a macro- and micronutleus hke the parent. The two 

 parental contractile vacuoles become the anterior vacuoles of the two 

 new individuals. The mouths of the young individuals arise by division, 

 of that of the parent. The original gullet persists as the gullet of the 

 anterior young individual, while an outgrowth from it forms that of the 

 posterior. 



The two new individuals so arising proceed with their growth and 

 presently repeat the process of fission. Under favourable conditions 

 fission takes place at more or less regular intervals (e.g. about once in 

 twenty-four hours) until it may be several hundred generations have 

 been produced. In time however the interval between successive fissions 

 becomes prolonged and eventually fission fails completely to take place. 

 This loss of the power to divide is accompanied by general enfeeblement 

 of the vital processes. There has come about a condition of senility or 

 depression which unless counteracted will lead to the death of the whole 

 culture. The process may be counteracted and new vigour given to the 

 culture by various stimuli — such as changes of food or other factors in 

 the environment — and if this corrective is applied in time the Paramecia 

 go on dividing with renewed vigour and the onset of senility is deferred. 



Amongst such antidotes to senescence the most important perhaps 

 in nature is the process of syngamy. At a period before the onset of 

 senescence the Paramecia develop a tendency to conjugate. Syngamy 

 does not take place readily between individuals of an isolated culture 

 such as has been described, in other words between closely related indivi- 

 duals. But if two broods are mixed together there comes about an 

 epidemic of syngamy when the individuals are ripe. After this has 

 taken place an isolated individual kept under favourable conditions 

 will be found to be rejuvenated and to have regained its full powers of 

 fission. 



The minute details of the process of syngamy in Paramecium are of 

 much interest owing to the striking peculiarity that the division of the 

 gametocyte into separate gametes has become suppressed except in so 

 far as the nuclei are concerned and the process of syngamy is no longer a 

 process of fusion of complete cell-individuals but only of nuclei. 



Two individuals (=gametocytes) become attached together by their 

 oral surfaces and in this position may be seen swimming about in a 

 normal manner. The micronucleus of each individual undergoes mitosis 

 twice. Of the four nuclei so arising three degenerate and no longer 

 function. The fourth on the other hand divides once again by mitosis 

 and the two nuclei so arising are the functional gamete nuclei. We 

 may take it that probably all four nuclei were once functional, each 



