PHYSIOLOGICAL DOMINANCE 91 



stitute rejuvenescence. The facts indicate that all 

 reproductive processes bring about rejuvenescence to 

 some degree, and it is certain that the new indi- 

 viduals which arise by division or budding from other 

 individuals or from experimentally isolated pieces are 

 to some extent physiologically younger than the parent 

 individual from which they arose. ^ Rejuvenescence in 

 such cases results from the loss of the differentiation as 

 a part in that portion concerned in the reproductive 

 process, and with the new individuation a new process 

 of senescence begins. 



Among the lower animals which have served as 

 material for the study of regeneration or regulation 

 two forms have been used to a large extent in my own 

 experiments and must be briefly described here. The 

 hydroid Tubularia in its simple unbranched form as 

 a single individual (Fig. 42) consists of hydranth, stem, 

 and stolon, the hydranth forming the apical end of the 

 stem and bearing two sets of tentacles, reproductive 

 organs between them, and a mouth at its apical end. 

 The stem grows vertically from the surface of attach- 

 ment, and the stolon adheres to the surface, forming an 

 organ of attachment, and elongates by growth at its 

 tip. Stem and stolon are covered by a horny cuticle, 

 the perisarc. The apical end of the metabolic gradient 

 of the major axis is the apical region of the hydranth, and 

 from this region the rate decreases basally through the 

 hydranth. In the stem the metabolic rate is lower than 

 in the hydranth, and there is a slight decrease in rate in 

 the basal direction, but at the growing tip of the stolon 

 there is a short, slight gradient in the opposite direction, 



^ Child, Senescence and Rejuvenescence, 1915. 



