REGENERATION I3I 



root that the cells are provided with two different kinds of idio- 

 plasm, which remain inactive until the adequate stimulus arises 

 and causes the idioplasm of either the root or of the stem to 

 become active. In both cases the loss of substance must be re- 

 garded as the stimulus, and the direction in which it acts must 

 decide the quality of the reaction. 



If the idioplasm of the tissue-cells were capable in itself of 

 responding to the effect of this stimulus by causing a regenera- 

 tion of the missing parts of the body, worms possessing the 

 regenerative power in a high degree, such as Nais and Liunbri- 

 ciilus, would be capable of regeneration in a lateral as well as in 

 the anterior and posterior directions. This, however, as Bonnet 

 has previously proved, is not the case : when cut in half longi- 

 tudinally, the missing right or left half is not reproduced, and 

 the cells of these animals must therefore be wanting in that sub- 

 stance — viz., in the cDitimeral supplementary determinants — 

 which renders this kind of reproduction possible. 



From our point of view, it is not surprising that these deter- 

 minants are absent in worms ; for in the natural state these 

 animals are never torn in half longitudinally, and there was 

 therefore no need for Nature to provide for such a contin- 

 gency. 



If we consider that the groups of supplementary determinants 

 must become more complicated in proportion as the organism 

 and the part to which they give rise increase in complexity, we 

 can understand \\\\\ facultative regeneration only occurs in rela- 

 tively simple organisms, and that it apparently takes place in 

 three dimensions in Polypes and Flat-worms only, in two dimen- 

 sions in Annelids and Starfishes, and merely in one dimension 

 in Arthropods, Molluscs, and Vertebrates. 



It must not be supposed that other factors do not also take 

 part in limiting the capacity for regeneration, — such as, in 

 particular, the vulnerability of the higher organisms, and the 

 fact that they are dependent on the circulation and tem- 

 perature of the blood, even apart from the influence of the 

 nervous system, of which we are practically still very ignorant. 

 The relatively small quantity of substance in the part removed, 

 as compared with that of the rest of the body, would also pre- 

 vent the amputated limb of a salamander, for instance, from 

 becoming regenerated into an entire animal. All these con- 

 siderations help to explain why bi-dimensional regeneration — 



