5 6 REGENERA TION 



after its removal from the rest of the body we have no direct means 

 of knowing whether or not the piece has potentially the power to 

 regenerate, but in some other cases, in which small pieces may be 

 kept alive for some time, they may not regenerate. Furthermore, 

 the regeneration of small pieces that are just above the minimal size 

 is often delayed and is sometimes imperfect. These small pieces 

 seem to meet with a greater difficulty in regenerating than do larger 

 pieces. Peebles has shown that pieces of hydra that measure less 

 than \ mm. in diameter (= about ^-<nr f tne volume of hydra) do not 

 regenerate, although if very small pieces are taken from a develop- 

 ing bud they may regenerate, even when only \ mm. in diameter. 

 Very small pieces that are, however, just above the minimal size, 

 while they may assume a hydra-like form, produce only one or two 

 tentacles. The failure of the smallest pieces to regenerate is not 

 due to their dying, since they may live for a much longer time than 

 would suffice for larger pieces to regenerate. Isolated tentacles of 

 hydra do not produce new hydras, although they may remain alive 

 for some time. A single tentacle is larger than the minimal piece, 

 so that its failure to regenerate is probably connected with the differ- 

 entiation of the tentacle, rather than with its size. The lack of 

 power to regenerate in the smallest pieces of hydra cannot be con- 

 nected with the absence of any special organ, 

 since these pieces contain both ectoderm and 

 endoderm. In tubularia also, Driesch and 

 I have found that pieces below a certain size 

 do not regenerate (Fig. 27). There is likewise 

 in planarians a lower limit of regeneration, 

 even for pieces that contain all the ele- 

 ments which, being present in larger pieces, 

 make regeneration possible. Lillie has found 

 that nucleated pieces of the protozoon stentor 

 FIG. 27. Tubularia mesembry- fail to regenerate if they are below the mini- 

 pce'"that p ro u n ceTa S ty d - mal size. He places this minimal size at 80 [i. 

 dranth B, c. Pieces below diameter, which he calculates as 9 V of the 



minimal size. D. Ring pro- * J 



duced by closing of small volume of the stentor from which the piece 

 has come. I have obtained a slightly smaller 



piece that regenerated, and since it came from a larger stentor it repre- 

 sents about g 1 ^ of the whole animal. The lack of the power of devel- 

 opment of these smallest pieces seems to be due to the absence of 

 sufficient material for the production of the typical form. We can 

 give no other explanation of the phenomenon at present, especially 

 since the pieces contain material that we know from other experiments 

 has the power of producing any part of the organism. The super- 

 ficial area of small pieces is relatively greater than that of larger pieces, 



