492 



ON INCREASE IN SIZE 



[PT. Ill 



US that Murray's law only applies to the embryonic period, and 

 must not be extended beyond it (see p. 548). A typical graph is 

 that shown in Fig. 73 where the composition of the whole human 

 being as regards water, nitrogen and ash, is considered. The point 

 of sudden cessation of intense chemical redistribution was termed 

 quite logically by Moulton "chemical maturity", and this point, he 

 found, bore a fairly constant relation to the total life-span, as appears 

 from the following table : 



Table 66. 

 Moulton's figures: 



Species 



Man 

 Cow 

 Pig ... 

 Guinea-pig 

 Dog 

 Cat ... 

 Rabbit 

 Rat ... 

 Mouse 



This, however, was the only relation which did show any constancy, 

 and very little can be deduced about an animal if only its gestation 

 time, or conversely, only its average length of life, is knov/n. 

 Even its composition at birth is not related simply to any of the 

 other variables. These facts, to which Moulton was the first to draw 

 attention, illustrate the truth of the statement just made, namely, 

 that the act of birth or hatching is a comparatively unimportant one 

 in the life of the individual. Probably the time at which it takes place 

 in the life-span has been much involved with the adaptations due 

 to different modes of life, while yet the underlying process of physico- 

 chemical maturation has remained unaffected. 



Extremely few researches have been done on these problems. In 

 1926 I estimated the non-protein nitrogen in a variety of bird 

 embryos, of different incubation periods, with a view to ascertaining 

 whether the rhythm of chemical differentiation went on at a constant 

 rate in all cases. The curve shown in Fig. 74 is from the data 

 on non-protein-nitrogen of White Leghorn embryos. Similar data 

 were obtained for two other races of domestic hen (21 days), the 



