268 SENESCENCE AND REJUVENESCENCE 



cell at a very high metabolic level. Consequently the degree of 

 dominance and of individuation may increase up to a certain point 

 as development proceeds. Moreover, the increasing differentiation 

 of the nerve fibers determines a more effective conduction of im- 

 pulses, and the increasing centralization of the nervous system and 

 complexity of nervous correlation results in a greatly increased 

 unity and co-ordination of the parts of the individual. It was 

 pointed out in chap, ix that the decrement in the conduction of 

 impulses in the nerves of the higher animals is scarcely appreciable 

 within the limits of the individual body. This means that in the 

 adult the limit of dominance, the physiological limit of individ- 

 uation, is far beyond the actual size attained by the individual. 

 Growth in these forms is limited by progressive differentiation, con- 

 sequently the final size of the individual remains far below the limit 

 of dominance in the differentiated nervous system, and the physio- 

 logical isolation of parts so frequent among the plants and lower 

 animals does not occur under ordinary conditions in the higher ani- 

 mals after the functional capacity of the nervous system has fully 

 developed. 



For the occurrence of agamic reproduction in differentiated 

 organisms the physiological or physical isolation of a part and 

 capacity of the part to react to isolation by regression and recon- 

 stitution are necessary. These conditions are not present in the 

 later stages of development of the higher animals, but isolation of 

 parts does occur to a limited extent in early stages of development 

 before the cells have undergone appreciable differentiation and be- 

 fore the individual has attained the degree of integration character- 

 istic of later stages. Consequently agamic reproduction in these 

 forms is limited to these stages. In a few species polyembryony 

 occurs as a normal feature of development, the egg undergoing 

 separation during cleavage or later embryonic stages into two or 

 more individuals. In certain parasitic insects, for example, indi- 

 viduation is apparently almost entirely absent during early stages 

 and, instead of developing in an orderly way as a single embryo, 

 the eggs as they divide separate repeatedly into cells or cell groups, 

 each of which finally gives rise to an embryo (P. Marchal, '04; 

 Silvestri, '06) . In these cases a single egg may give rise to a large 



