384 SENESCENCE AND REJUVENESCENCE 



factors which decrease metabolism we do not know. It may be 

 that such processes as budding and the replacement of differ- 

 entiated old cells by young cells from the interstitial tissue prevent 

 progressive development in hydra beyond a certain point under 

 the usual conditions, in which case low temperatures by decreasing 

 the rate of metabolism may bring about essentially the same changes 

 that would occur in further development determined by internal 

 factors. In many if not in all of the coelenterates, however, there 

 are indications that the formation of gametes is associated with an 

 advanced stage of the life cycle. 



One of the most interesting cases is that of certain jelly-fishes 

 or medusae belonging to the family Margelidae (Chun, '96; Braem, 

 '08). These medusae reproduce agamically by budding, and buds 

 arise in a definite order upon the proboscis and develop from the 

 ectoderm alone instead of from both body layers, as do other coelen- 

 terate buds. The young medusa gives rise to these buds, but as it 

 grows older sex organs begin to appear from the same body layer 

 and in the same region as the buds and sometimes in place of them. 

 Fig. 196 shows the proboscis of one of these medusae on which 

 both buds and ovaries containing eggs are present. After the sex 

 organs once appear the buds gradually cease to form and only 

 gametes are produced in later stages. 



In these medusae the same region and layer of the body and, 

 so far as we can determine, cells of exactly the same character, give 

 rise in the younger animal to agamic buds and in later stages to 

 gametes. Very evidently the physiological condition of these cells 

 undergoes change during the life history of the animal. Braem 

 regards this case as indicating that the agamic buds as well as the 

 gametes arise from germ plasm, but it seems rather to indicate that 

 gametes as well as agamic buds may arise from cells which are 

 functional, more or less specialized parts of the organism, and that 

 the gametes are more highly specialized cells and arise later in the 

 life history than those which develop into buds. 



As regards the planarian worms a few facts are at hand. Atten- 

 tion has already been called (pp. 99, 125) to the fact that Planaria 

 dorotocephala is not known to reproduce sexually at all in the locality 

 where I have collected material. But in stocks which are kept in 



