I20 THE BIOLOGY OF STENTOR 



5. Minimum size of regenerating fragments 



Given at least one macronuclear node, how small may a frag- 

 ment be and still regenerate? In the earliest cutting experiments 

 on stentors, Gruber (1885b) had already found that not only halves 

 and thirds but even smaller fragments of coeruleus regenerate and 

 form tiny Stentors. The embryologist Lillie (1896) raised the 

 question of the limits of divisibility of stentors as leading to 

 significant theoretical implications. Fragmenting the ciliates by 

 shaking, he found that no piece smaller than i/24th the volume of a 

 large polymorphiis regenerated completely, and the minimal size 

 for coeruleus w^as i/3oth. Lillie was impressed by the fact that such 

 fragments are still of considerable size, since they were about 80 ju, 

 in diameter, and therefore emphasized that the cytoplasm is as 

 important as the nucleus to regeneration, postulating that there 

 is a ** minimal organization mass" below which the complete, 

 potential form of Stentor could not find representation. This size 

 limit should be absolute rather than relative ; therefore he expected 

 that it would not be exceeded even if one started with smaller cells 

 for cutting. Morgan (1901a) found that pieces no larger than 

 I /64th of the whole coeruleus could regenerate and this was later 

 confirmed by Stolte (1922). Morgan's minimal fragments were in 

 fact only slightly smaller than Lillie's but they were cut from 

 larger cells. Recalling that there are also lower limits to the size of 

 regenerates in Hydra, Tubularia, and Planaria, Morgan offered a 

 first-order explanation for the failure in regeneration of very small 

 pieces in both metazoa and ciliates, namely, that there is simply 

 insufficient material to produce the typical form. 



Sokoloff (191 3) pursued this problem further in the ciliates 

 Dileptus and Spirostomum. The first is suitable because the macro- 

 nucleus is finely subdivided and widely distributed, and the second 

 because the very elongate shape lends itself to cutting tiny frag- 

 ments. Pieces i/8oth the volume of the whole cell could regenerate. 

 Although fragments i/iooth of the normal size could be cut, these 

 did not regenerate or survive for long. Therefore Sokoloff (1934) 

 seems to have settled on the idea that there is really no theoretically 

 significant limit to the divisibility of ciliates, and that in practice a 

 limit is imposed only by the circumstance that in smallest frag- 

 ments the wound surface with its exposed endoplasm is relatively 

 ^-SoJ-ajcge that the pieces become vacuolated and soon disintegrate. 



