PROCESS OF REPRODUCTION IN ORGANISMS. 15 



nucleus itself becomes differentiated and chromatin disappears 

 from it completely or almost completely. If such a cell as this 

 were found elsewhere in the body and had no reproductive 

 function there is not the slightest doubt that we should regard 

 it as a very highly differentiated and specialized cell. 



The spermatozoon is certainly not an undifferentiated cell 

 according to the usual criteria. It is one of the most highly 

 differentiated cells that we know of in the organism. Its differ- 

 entiation has proceeded along different lines from that of the 

 egg cell, but it is none the less differentiation. Even the chro- 

 matin is in a condition different from that in any other cell. 



What evidence is there that either of these cells contain "un- 

 differentiated germ plasm?" If we confine ourselves to facts 

 they are among the most highly differentiated cells known. 



Morphologically then the gametes are highly differentiated 

 cells. What is their physiological condition? Are they physio- 

 logically old or young? As noted above, my experiments with 

 Planaria and other forms have led me to the conclusion that 

 senescence consists morphologically in the accumulation of 

 relatively inactive substances or in an increasing density or im- 

 permeability or more strictly a decreasing capacity of the colloid 

 membranes to become permeable under stimulation as a necessary 

 result or incident of continued metabolism under relatively 

 constant conditions and in the presence of nutritive material. 

 Usually the inactive substances appear as more or less stable 

 structural features, as differentiations, and their accumulation 

 or increasing density presents increasing obstacles to metabo- 

 lism. Physiologically then, senescence consists in a decrease in 

 the rate of metabolism in consequence of the increase of struc- 

 tural obstacles to metabolism. The cell becomes less irritable, 

 less capable of stimulation, and if the process goes far enough 

 death or quiescence is the result. 



This view of senescence is somewhat different from the theory 

 recently put forward by Minot (Minot, '08). For Minot 

 cytomorphosis, i. e., the increase in cytoplasmic as compared 

 with nuclear material is the important factor in senescence. 

 The two views have, however, much in common, for the accumu- 

 lation of structural substances usually means increase of cyto- 



