VARIATION 423 



or a Hydroid multiplies asexiially, the separated-off piece 

 shows marked rejuvenescence as revealed by the two tests 

 named. 



Professor Child's thesis is this: As an organism differen- 

 tiates, it ages, for the accumulation of relatively inactive con- 

 stituents in the colloidal cytoplasmic substratum necessarily 

 involves a decrease in the metabolic rate ; but there are coun- 

 teractive processes of reduction, removal, and de-differentia- 

 tion, when the metabolic stream erodes its bed instead of 

 depositing materials. These are marked by acceleration in 

 metabolic rate, and constitute rejuvenescence. '' It is cer- 

 tain," Professor Child says, " that the new individuals which 

 arise by division or budding from other individuals or from 

 experimentally isolated pieces are to some extent physiologi- 

 cally younger than the parent individual from which they 



arose." 



The idea of a see-saw between processes of senescence and 

 rejuvenescence finds many illustrations among the lower 

 animals, but what of higher levels? Professor Child finds 

 some interesting evidence that the early developmental stages 

 of a number of animal types, before specialisation of cells sets 

 in, are conspicuously young in the physiological sense. The 

 germ-cells themselves are very stable condensations of he- 

 reditary items, but in the early development there is a time 

 of re-constitution, of de-differentiation, of relaxation. If 

 there is any soundness in this view, in support of which data 

 are, of course, submitted, we may perhaps recognise another 

 opportunity for variation, namely in the very young embryo, 

 Avhere the alleged rejuvenescence may include possibilities 

 of re-arrangement and, as it were, re-tuning. 



