120 INDIVIDUALITY IN ORGANISMS 



ganglion and so to a head, while they may still be able 

 under the dominance of other parts to produce basal 

 regions of the body. The development of the apical 

 region or of the apical part of the central nervous system, 

 which in all except the lowest animals is the primary and 

 dominant part of the apical region, is a self-determining 

 process, independent of other parts, while the develop- 

 ment of other parts is determined by their relations 

 to dominant regions. It is highly probable therefore 

 that a more complete loss of differentiation is necessary 

 as a condition for head-formation than for the develop- 

 ment of other parts. As a matter of fact, we find that 

 as the capacity for reconstitution becomes limited by 

 increasing differentiation the capacity for head-formation 

 disappears first of all. Many animals in which recon- 

 stitution of new heads does not occur are still able to 

 reproduce all subordinate parts, and with further limita- 

 tion it is the more subordinate parts, such as legs and 

 other appendages or caudal regions, which the body is 

 capable of reproducing. 



This limitation is more or less progressive from lower 

 to higher forms, until in the higher vertebrates the 

 capacity for reconstitution under any known con- 

 ditions is limited practically to tissue regeneration. 

 The primary limiting factor is unquestionably the 

 increasing physiological stability of the protoplasmic 

 substratum, in consequence of which the capacity for 

 dedifferentiation and rejuvenescence, at least under 

 ordinary conditions, is more and more narrowly 

 limited. 1 



1 Child. Senescence and Rejuvenescence, 1915, pp. 35, 39, 41-43, 53, 

 194, 267, 304, 463-65- 



