THE CELLULAR CHANGES OF AGE 83 



corpuscles. One of the most familiar instances of des- 

 truction is that of the tail of the tadpole. The young- 

 frog and the young toad during their larval stages live 

 in the water and each of them is furnished with a nice 

 tail for swimming purposes. As the time approaches 

 for the metamorphosis of the tadpole into the adult, the 

 tail is gradually dissolved away. It is not cast off, but 

 it is literally dissolved, resorbed, and vanishes ulti- 

 mately altogether. 



It is evident that such a vast amount of destruc- 

 tion of living cells could not be maintained in the 

 body without the body going entirely to destruction 

 itself, were there not some device for making good 

 the losses which are thus brought about. We find 

 in fact that there is always a reserve of cells kept to 

 make good the loss which it is essential should be 

 made good. Some losses apparently do not have 

 to be repaired, but the majority of them must be 

 compensated for, and this is done by having in the 

 body a reserve supply of cells which can produce new 

 cells of the sort required. This leads us to considera- 

 tion of the phenomenon of regeneration and of the 

 repair of parts. These phenomena we can better 

 take up later in our course, after we shall have dealt 

 with the general processes of development and 

 growth. From the study of regeneration we shall be 

 able to confirm the explanation of old age, which I want 

 to lay before you. This confirmation is so important 

 that it will be better taken up in a separate lecture, 

 than slipped in now when the hour is nearly by. 



