352 SENESCENCE AND REJUVENESCENCE 



the same progressive change in susceptibility, although it is very 

 difficult to determine with certainty when death occurs in the 

 mature spermatozoon. 



In the other forms examined attention has been directed chiefly 

 to the female cells, because the different stages of development are 

 readily distinguishable by size and because in the male the cells are 

 minute, the different stages being in most cases less readily dis- 

 tinguishable in the living cells, except under very high powers, and 

 finally the spermatozoa are motile and it is practically impossible 

 to eliminate the motor activity without injuring the sperm or 

 altering its physiological condition. In all cases where female 

 cells were examined the results are similar to those with Ascaris 

 cells. The susceptibility of the primitive mother cells is high, 

 approaching that of embryonic cells, and decreases progressively 

 during development of the gamete, and that of the full-grown egg 

 is exceedingly low lower than that of most differentiated cells. 

 Wherever the stages of spermatogenesis could be clearly distin- 

 guished the same results have been obtained for the non-motile 

 stages. 



The susceptibility to cyanide of conjugating infusoria (Colpid- 

 ium) is very distinctly lower than that of non-conjugating and divid- 

 ing stages (see p. 381). The conjugating stages in these animals 

 are comparable to the fully developed gametes of multicellular 

 forms, and their low susceptibility indicates that their rate of 

 metabolism is lower and they are physiologically older than other 

 stages. 



If the susceptibility method can be trusted, and a large and 

 increasing volume of evidence indicates that it can, the development 

 of the gametes in animals is associated, as the decrease in suscep- 

 tibility indicates, with a decrease in rate of metabolism a process 

 of senescence and the fully developed gamete is physiologically 

 an old cell approaching death. 



Chemical analysis of heads of spermatozoa, 1 so far as it throws 

 any light on the question, indicates that at least some spermatozoa 



1 The literature of the subject, including the pioneer work of Miescher and A. P. 

 Mathews' analyses (Mathews, '97), is discussed by Burian, '04, '06. Recently 

 Steudel ('na, 'n&, '13) has made new analyses with improved methods. 



