THEORIES OF POLYEMBRYONIC DEVELOPMENT 87 



and is not found in the eggs of other mammals. Whether 

 the structure is the cause of retarded development, or 

 merely one of its results, cannot be decided at pres- 

 ent. In the paper just referred to I made the fol- 

 lowing statement, which deserved, I believe, further 

 elaboration: 



According to Professor Child's theories of development and 

 reproduction, any part of a system, which, through a lowering 

 of the rate of metabolism of the controlling part of the system, say 

 the animal pole of the blastodermic vesicle, is liable to physiologi- 

 cal isolation of [subordinate'] parts at certain distances from the 

 dominant region. When such isolation of [subordinate] parts 

 occurs, new centers of control arise, which produce outgrowths 

 [apical ends] capable of establishing new systems like the original. 



A familiar instance of agamic reproduction in plants 

 will serve to make this theory clear. In a plant the 

 growing tip (apical end) is the dominant end of the 

 branch and it seems to hold in physiological subordina- 

 tion a considerable part of the branch distal to itself. 

 If, however, anything happens to this growing tip 

 (apical end) which lowers its rate of metaboHsm, a 

 whirl of secondary growing tips will appear just back 

 of the original apical end, and each of these new apical 

 ends will become new dominant regions, capable of 

 producing new individuals.^ Any type of environ- 

 mental change that has the effect of lowering the rate 

 of metabolism of the plant will have the effect of 



' The bracketed words in this quotation were not in the original but 

 are inserted here for the sake of clearness. 



' In plants the individual is not so sharply defined as in most 

 animals. For our purposes we may consider each growing point an 

 individual and the whole plant a colony. 



