1 8 C. M. CHILD. 



substances in the secretion or in the water which surrounds 

 the spermatozoon. Synthesis is apparently impossible for the 

 isolated spermatozoon: its activity is limited to one highly 

 specialized type. Meyerhof ('n) has recently shown that in 

 the sea urchin, Strongylocentrotus lividus, the respiration of a 

 certain quantity of sperm at 19 C. decreases about thirty per 

 cent, in four hours after removal from the body. This fact 

 indicates at any rate that the metabolism of the spermatozoon 

 decreases after its isolation from the body. 



I believe that the spermatozoon represents an extreme case of 

 that type of senescence which is associated, not with the accumu- 

 lation of actual volume of substance but with increasing density 

 or impermeability. In no other celt is the nuclear substance 

 in so dense a form and certainly this condition is the most un- 

 favorable conceivable for metabolism, and in no other cell is the 

 cytoplasm so completely converted into stable structural ele- 

 ments. 



If these conclusions are correct then the egg and the spermato- 

 zoon in their most highly differentiated forms represent the two 

 extreme types of the process of senescence. Attention must, . 

 however, be called to the fact that some eggs and some spermato- 

 zoa are much more highly differentiated than others. In many 

 of the smaller eggs increasing impermeability of the membranes 

 may be as important a factor in senescence as the increase in 

 actual volume. In some spermatozoa, on the other hand, con- 

 siderable cytoplasm remains. But the important fact in all 

 cases is that both gametes are highly differentiated cells and that 

 their structural characteristics, whether these consist in accumula- 

 tions of inactive substance or of impermeable membranes or of 

 high density, have apparently reduced their metabolic activities 

 to a minimum and made synthesis impossible. In short, both 

 gametes appear to be in an advanced stage of senescence and 

 both as isolated cells are approaching inevitable death. Cer- 

 tainly there is no indication that they consist of or contain 

 undifferentiated germ plasm. 



