SENESCENCE IN HIGHER ANIMALS AND MAN 269 



number of individuals. In the nine-banded armadillo the embryo 

 begins development as a single embryo, but later undergoes recon- 

 e-titution into four embryos by a process of budding (Patterson, '13). 

 In other species of armadillo a similar process of embryonic repro- 

 duction undoubtedly occurs. The cases of duplicate twins and 

 various forms of double monsters are probably also cases of embry- 

 onic reproduction from a single egg (Wilder, '04), but it is not 

 certainly known at what stage the reproduction occurs. 



In addition to the occasional occurrence of polyembryony the 

 ] process known as segmentation occurs as a characteristic feature 

 of development in all the higher animals, both invertebrates and 

 vertebrates. Segmentation, however, is rather a repeated indi- 

 viduation of parts from embryonic tissue than a reproduction from 

 differentiated cells, and does not therefore involve any considerable 

 regression and reconstitution. The segment-individuals which arise 

 in succession as morphogenesis proceeds posteriorly along the axis 

 (see Figs. 70, 197, 198) never complete development to whole 

 animals, but remain as segments subordinate to the dominating 

 head-region. Aside from these cases of polyembryony and repeti- 

 tive formation of segments, agamic reproduction plays no part in 

 the normal life history of the higher animals, and it is evident that 

 these reproductions, since they occur so early in development, can 

 have but little significance in bringing about rejuvenescence or 

 retarding the progressive course of senescence. 



A most important consequence of the stability of structure and 

 the absence of agamic reproduction in these animals is the greater 

 continuity of progressive development and senescence. In the 

 lower forms progressive development may be interrupted repeat- 

 edly, or even periodically completely compensated, by agamic 

 reproduction of one kind or another with its accompanying rejuve- 

 nescence. Where such reproduction is absent the regressive changes 

 may occur to some extent in tissue regeneration, in the periodic 

 elimination of previously accumulated material in gland cells 

 (see pp. 189-91), or during starvation, and under certain other 

 conditions which bring about excessive structural breakdown, but 

 such changes are either narrowly localized and without appre- 

 ciable effect upon the body as a whole, or they are so slight that it 



