The Biology of Senescence 



fifth month. Somatic mitosis in adult life occurs r only in the 

 mid-gut of Cyclops, and in this region 'senile' changes were not 

 found (Harms, 1949). The most striking degenerative changes 

 in Cyclops were found in the chromatin of the ganglion cells, 

 with the appearance of large inclusion bodies suspiciously 

 reminiscent of virus inclusions or fixation artefacts, and in the 

 antennules. Gut degeneration occurred later in life, about the 

 eighth month, and was confined to the anterior gut, mitosis con- 

 tinuing in other parts of the gut epithelium until the end. It is 

 difficult to know what connection, if any, these changes have 

 with the process of senescence. 



Needham (A. E. Needham, 1950) has studied the growth 

 rate of limb regeneration in Asellus aquaticus; growth in Crus- 

 tacea, on Needham's figures for Asellus and Carcinus, is deter- 

 minate and the curve sigmoid, the arithmetic rate of growth 

 rising to a maximum and declining asymptotically there- 

 after to zero. The geometric rate of growth declines monoton- 

 ically with age from the outset, and the rate of decline itself 

 declines with increasing age. The specific regeneration rate 

 decreases progressively with age, owing to the progressive 

 increase in the duration of each instar; the rate of decline is 

 much less than that of the normal growth rate, and itself de- 

 clines with age. 'In some Crustacea the limiting size is attained 

 at an age beyond the mean expected life-span. Growth is 

 indeterminate in Crustacea only in this sense. They are not 

 potentially immortal.' This investigation illustrates once again 

 the difficulty of characterizing the growth-behaviour of real 

 organisms mathematically: the decline of arithmetic growth 

 may be asymptotic, or tangible size increase may continue; 

 in Crustacea there is the additional difficulty that growth is dis- 

 continuous, being interrupted by stadia, which superimpose a 

 'quantal' effect on the smooth ideal curve. The conclusion of 

 Needham's studies is that the growth of Crustacea follows a 

 convergent series, and must cease, presumably, for practical 

 purposes in any form which lives long enough. But the relation 

 between such cessation and determinacy of life-span is still 

 entirely conjectural. 



Insects. It has long been recognized that several separate 

 types of senile change may occur in insects. Mechanical damage 



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