102 Regulation of Size in Unicellular Organisms 



('63) found that, in Gromia, pseudopodia of the same 

 individual would fuse with one another, but not with 

 those of different individuals. Kepner and Reynolds 

 ('23) found for various species of Difflugia that pseu- 

 dopodial fragments cut off from an individual were not 

 only reincorporated with the cytoplasm from which 

 they came, but that the individual reacted toward the 

 fragments in such a way as to creep in their direction. 

 When fusion between individuals did not occur, there 

 was, on the contrary, a shattering of the protoplasmic 

 surface. Fragments from one individual were some- 

 times incorporated by certain other individuals; prob- 

 ably they belonged to the same clone. In Arcella, Rey- 

 nolds ('24) showed clearly that the hereditary relation- 

 ship among individuals determined whether the frag- 

 ments would fuse or shatter. Starting from a single 

 parent, the descendants became gradually diverse 

 enough so that fusion occurred rarely or not at all. 

 Diverse environments were effective in hastening the 

 attainment of diversity in the cytoplasm. 



Among ciliates, where most is known about size- 

 regulation and pedigree's, the instances of cell fusion 

 are particularly rare. This in itself may be an evidence 

 of more efficient control of size. Illustrative of ex- 

 periments which have been tried are those of Prowa- 

 zek ('01), who obtained very temporary fusions in re- 

 sponse to cutting in Glaucoma; and those of Morgan 

 ('01) who transplanted without success cut pieces of 

 Stentor. Greater hope of success in grafting lies in 

 first modifying the physical properties of the cyto- 

 plasm by radiation (MacDougall, '29) or by chemical 

 conditions (Heilbrunn, '27). 



Significance of fusions. In order to evaluate fully 

 the significance of an observed or induced fusion of 

 individuals, one must know the genetic relationship 

 of the individuals, and then follow the individuals and 



