REJUVENESCENCE IN UROLEPTUS MOBILIS 141 



cirri. In Paramecium caudatum ('04) I found that the micro- 

 nucleus is degenerated through hypertrophy, while the cortex 

 showed degeneration mainly through the entire absence of 

 trichocysts. Similarly, the last individuals of a series of Uro- 

 leptus mobilis show evidences of morphological as well as physi- 

 ological degeneration. They become greatly reduced in size 

 and the micronuclei entirely disappear, probably by absorption. 

 The macronuclei do not disappear, but show characteristic 

 changes indicating degeneration. The eight separate nuclei 

 usually remain independent, but show evidence of an attempt to 

 fuse as they do prior to division in a normal individual. Their 

 chromatin contents, however, are quite different from the normal; 

 instead of an equal distribution of uniform granules, it masses 

 together to form an intensely staining nuclear body similar to 

 those which form from the degenerating macronuclei subsequent 

 to conjugation (fig. 1, 2, 3, 4', cf. fig. 87, and 89, Calkins, '19). 



Unlike Maupas's hypotrichs, Uroleptus does not show cyto- 

 plasmic changes further than reduction in size : the membranelles 

 and cirri do not disappear and the contractile vacuole keeps up 

 its activity. 



In this degenerate condition the last individuals of a series 

 are unable to divide — division, apparently, being impossible 

 without a micronucleus. They sometimes show a remarkable 

 tenacity of life, however, in this final phase, indicating that 

 metabolic activities may go on despite degeneration. Thus the 

 last individual of the A series lived for thirteen days after the 

 final (313th) division: the last of the D series for eighteen days; 

 the last of the G series for fourteen days; the last of the H series 

 for fifteen days. The most remarkable cases of longevity of the 

 single individual occurred in the C and the B series. The last 

 individual of the C series lived in its 348th generation for thirty- 

 six days without dividing again, and the last one of the B series, 

 formed as a product of the 257th division on August 31st, lived, 

 without dividing again, until October 9th, a period of forty days. 

 These last individuals, like all others, were transferred daily to 

 fresh standardized culture medium, and their death was not due 

 to violence. 



