954 GROWTH AND THE AGING PROCESS I4 



ses of decline, with which we associate senescence, begin to appear. This objection, 

 however, may be more apparent than real. The line of division between the up- 

 ward slope of growth and the downward one of senescence may not be a sharp one. 

 Perhaps the cessation of growth, while the actual initiating factor in senescent 

 change, cannot make its effect felt except over a period of some time, varying in 

 different species. 



If we turn to the lower animals, we find that very cogent evidence has been 

 obtained that some sort of "aging potential" does begin to accumulate at the time 

 of cessation of growth (Lansing, 1947a). This "aging potential" of the organism 

 is demonstrable in an indirect manner. The organism studied was the rotifer, a 

 minute but multicellular animal, with a reproductive process which often is 

 parthenogenic through many generations. Lansing found that the offspring from 

 parents who were beyond the growth period showed a life-span of shorter duration 

 if the parents were older at the time of production of the egg. Rotifers still actively 

 growing appeared to be completely free of this aging potential. Of the results of 

 these studies, Lansing says (1947b, p. 333): "The evidence obtained by Lansing 

 supported the view that aging does not, as commonly believed, begin in the fertilized 

 ovum; on the contrary, aging appears to be a by-product of changes which occur in 

 the cell at the time of growth cessation". 



The term orthoclone is used by Lansing to designate a series of generations with 

 constant parental age through the generations. Thus a "senile orthoclone" is one in which 

 the eggs used for rearing the individuals are taken always from old mothers. 



In such an orthoclone the mean life span of each successive generation can be 

 decreased until the point of extinction is reached (Fig. i). This trend can be 

 stopped and actually reversed by altering the selection technique, simply choosing 

 young mothers rather than old. 



In contrast, the mean life span of successive generations increases progressively 

 in a young or adolescent orthoclone. In one experiment which lasted through 49 

 generations the span was lengthened to 1 00 days as compared with 24 days for the 

 ancestral stock. 



It should be stressed that the "aging potential", as measured by these experi- 

 ments, is present not only in senile rotifers but in all maternal animals which are 

 not actively growing. 



Rate of sexual maturation bears an inverse relation to longevity. As longevity 

 decreases in an "old" orthoclone, the age of sexual maturation, measured by the 

 beginning of egg-laying, becomes progressively less; while as longevity increases 

 in a "young" orthoclone, the age of sexual maturation becomes increasingly 

 greater. Thus an intimate relation exists between growth and longevity and be- 

 tween rate of sexual maturation and longevity. 



Do similar relationships occur in animals other than the relatively primitive 

 rotifer? A good deal of evidence exists that they do, and this evidence extends 

 even to the mammals (MacDowell and Taylor, 1948; Strong, 1948; and Strong, 

 1950). 



The growth curves of a number of different species of warm-blooded animals 

 bear a surprising resemblance to each other and in fact can be made to coincide. 

 Man differs from other species in showing a very long "juvenile period'' between 



