GENERAL CONSIDERATIONS 35 



division until there may be thousands in the container. If conditions are 

 appropriate, this massing may be followed by an epidemic of con- 

 jugations (see Sonneborn, /nfra, Chapter XIV). In this process two 

 individuals fuse at the anterior ends and remain united for approxi- 

 mately twenty-four hours ( Fig. 13). They then separate and each of 

 the ex-con jugants begins a process of reorganization which lasts for 

 four or five days. If such an ex-conjugant is isolated and fed, it will 

 give rise to a new series and its progeny will pass through all the stages 

 of vitality that the parent series passed through. At the outset its vitality 

 will be approximately the same as the original vitality of the parent 

 series, but it will be greater than that of the parent series at the time 

 of conjugation. It passes through the same history of waning vitality, 

 and its protoplasm finally dies, although this may be months after the 

 death of the parent protoplasm. 



These experiments with Uroleptus, continued for more than ten 

 years and with 146 different series, always gave the same result. The 

 length of life of each series varied from three months to over a year 

 and all were descendants of a single bit of protoplasm making up the 

 body of one Uroleptus cell. 



There is little reason to doubt that some change in the protoplasmic 

 make-up occurs with continued metabolic activity through many suc- 

 cessive generations by division. In isolation cultures it is manifested by 

 an increasing time interval and by a change in the physical properties 

 of the protoplasm whereby fusion of cells, total or partial, is possible. 

 A type of breakdown, not manifested early in the series, is set up. The 

 cortex is liquified, in most cases in the region of the peristome, and two 

 individuals fuse in conjugation. In some cases the entire cortex is thus 

 modified, and individuals will fuse at any point on the periphery. I 

 have often referred to one case where no less than nine Paramecium 

 caudatum were thus fused into one amorphous mass. 



The stimulus of normal fusion results in activities not manifested 

 before. The micronuclei divide as they do in endomixis, and their 

 division involves reduction in number of chromosomes and in the 

 formation of pronuclei which meet and fuse. These copulating micro- 

 nuclei, as pronuclei, are usually interpreted as a reminiscence of an 

 ancestral gamete brood which is realized in Dallasia [Glaucoma) fron- 

 tata, Opalina, and in gregarines. In the latter, as is well known, each indi- 



