78 BIOLOGY OF DEATH 



cent as a whole ; that is, has undergone a physico-chemi- 

 cal alteration (as compared with that of a young ani- 

 mal), which is comparable to the morphological and 

 physiological changes which are observable in senescent 

 cells. It may further quite reasonably be supposed that 

 "senescent" serum, because of these physico-chemical 

 alterations, does not furnish so favorable a nutrient me- 

 dium for in vitro cultures as does "young" serum. Such 

 a view avoids the necessity of postulating a specific 

 "senescent" substance, the existence of which would be 

 exceedingly diflScult to prove. 



But in any case, whatever explanation is suggested 

 for Carrel and Ebeling's brilliant results, it does not 

 seem to me that the results themselves, which alone are 

 the realities pertinent in the premises, either offer any 

 obstacle to or, indeed, alter the interpretation of senes- 

 cence which I have suggested above- For, what the re- 

 sults really demonstrate is, essentially, that the serum of 

 old animals is a less favorable component of the nutrient 

 medium of cells in vitro than is the serum of young ani- 

 mals. This fact is a contribution to our knowledge of 

 the phenomena and attributes of senescence of first-class 

 importance ; but it does not per se, as it appears to me, 

 permit of any new generalization as to the etiology of 

 senescence. 



