REJUVENESCENCE IN UROLEPTUS MOBILIS 155 



ments to the contrary in connection with other cihates, and it 

 may well be that this condition of sexual immaturity noticeable 

 in Uroleptus is not universal among ciliates. 



As Maupas found for other hypotrichous ciliates, the condi- 

 tion of physiological depression is accompanied by morpho- 

 logical changes. During the period of active metabolism the cell 

 rapidly grows to full size after division (140^ to 165 ^u). The 

 macronuclei are eight in number, with from four to six micro- 

 nuclei. In preparation for division a portion of each macro- 

 nucleus is thrown off and is absorbed in the cytoplasm, while the 

 remaining portions fuse to form a single division nucleus. All 

 but two of the micronuclei are likewise absorbed. In the late 

 individuals of a cycle, the macronuclei lose their characteristic 

 nuclear clefts and in some cases show a tendency to fuse, while in 

 other cases the number is increased from eight to as many as 

 sixteen smaller and irregularly shaped nuclei. The micronuclei 

 do not increase in number, but undergo degeneration by hyper- 

 trophy or by granular degeneration, and finally disappear in the 

 cytoplasm. In the last individuals of a series, the chromatin of 

 each macronucleus collects in a single large, highly refractile, 

 and densely staining granule. The size of the cell is greatly 

 reduced and it is unable to divide. The cytoplasm is probably 

 as much changed as the nuclei, but morphological evidence of 

 such change is difficult to detect. In general there is a tendency 

 to increased vacuolization, while the mitochondria, which form 

 a cortical layer in the normal individual, are rare and irregularly 

 distributed. Some individuals in this final stage of depression 

 live without dividing for thirty-six (C series) and forty (B series) 

 days. 



In conjugation, apart from the processes of maturation and 

 reduction in number of chromosomes from eight to four, and 

 union of the gametic nuclei, the most significant phenomenon is 

 the granular disintegration of the old macronuclei, and absorp- 

 tion of the relatively large quantity of nuclear substance in the 

 cytoplasm. Not only do the macronuclei thus furnish nucleo- 

 proteins to the cytoplasm, but the micronuclei also contribute no 

 small part. Thus, if an individual goes into conjugation with 



