INDIVIDUATION AND REPRODUCTION 229 



isolated parts are capable of reacting to tlie change, first, in a loss 

 of their characteristics as parts, and, secondly, if conditions permit, 

 in a new individuation which may bring about the development of 

 a complete new individual from the isolated part. In short the 

 isolation of a subordinate part from the influence of the dominant 

 part is a necessary condition for reproduction. In experiment 

 pieces are physically isolated from the body of the animal by section, 

 and in the lower simpler forms reproduction follows such isolation, 

 and the piece becomes a new whole, or at least undergoes changes in 

 that direction. 



There are certain features of the simpler reproductive processes 

 in nature which suggest that in these cases, as in the experimental 

 reproduction of artificially isolated pieces, an isolation from the 

 influence of the dominant part is the essential condition for repro- 

 duction. In many forms, both plants and animals, growth beyond 

 a certain length or size, which is dependent upon rate of metabohsm, 

 degree of differentiation, etc., results in the transformation of that 

 portion of the individual most distant from the dominant part into 

 a new individual. The case of Tubularia mentioned above (Fig. 94, 

 p. 220) is a good iUustration, and in many plants similar vegetative 

 reproductions occur. It is impossible to doubt that in such cases 

 growth to a certain size brings the region in question into a condi- 

 tion where it is able to behave as if it were physically isolated, 

 Hke a piece cut from the body. 



It is also a fact, however, that reproduction may occur in conse- 

 quence of the weakening or removal of the dominant part and with- 

 out any preceding increase in size of the individual. Such cases 

 are very common among the plants, where the removal or inhibition 

 of metabohsm of the growing tip of the main axis or stem is fol- 

 lowed by development of a new axis from a lateral branch or bud. 

 Very commonly also the removal of all growing tips is followed by 

 the development of "adventitious" growing tips, which often arise 

 from differentiated cells by a process of dedifferentiation and growth. 

 Among the lower animals similar cases occur. Increase in size is 

 not then a necessary condition for reproduction. Decrease in rate 

 of metabolism or inhibition of metabohsm in the dominant region 

 may bring about reproduction as readfly as growth. 



