SECT. 3] AND ORGANISATION 547 



and form would therefore seem to be what Murray calls "aligned" 

 with one another, for the most marked form changes occur at the 

 initiation of embryonic development when the growth-rate is at its 

 highest. Murray's interim method affords a basis of quantitative 

 comparison between growth-rate and differentiation-rate. But if 

 these two processes change together with like velocity, others do not. 

 The shifts of chemical and physical change within the embryonic 

 body follow an entirely different course In 1925 I had drawn 

 attention to the fact that, making allowance for the increase in 

 size of the embryo and for its consequently increased daily turn- 

 over of matter and energy, chemical activity seemed to be more 

 intense at the end of development than at the beginning. "One 

 pictures", I had said, "the gradual elaboration of structure up to 

 the beginning of the third week, followed then by a burst of chemical 

 activity consequent upon the assumption of function by the organs 

 already formed. In this hypothesis is involved the view that in the 

 first fortnight chemical change is limited to those compounds which 

 are required mainly for structural purposes, while toward the end 

 of incubation the opening up of functional operations causes marked 

 and profound chemical changes of other kinds." This was proposed 

 in connection with the three periods which modern embryologists 

 have come to recognise in all embryonic development. The same 

 idea was elaborated further by Murray in relation to change in 

 growth-rate and differentiation-rate. Taking the rate of change in 

 chemical constitution, he concluded (largely from his own experi- 

 ments) that the most marked changes occurred after the loth day, 

 not before it, as is the case with growth-rate. "Internal integration", 

 he said, "may be regarded as a process characterised by the con- 

 centration of solid substances within the body, whereas chemical 

 differentiation is a change in the composition of the solid substances 

 thus integrated." Fig. 92, taken from Murray, demonstrates these 

 relationships, for it may be observed that the percentage of ash in 

 the embryo, the percentage of total solids and the percentage of fat, 

 together with the rate of production of carbon dioxide, all rise or 

 fall in the same way, i.e. more rapidly at the end of development 

 than at the beginning, and precisely opposite to the growth-rate and 

 the differentiation- rate. Thus, just as morphological growth and 

 differentiation take place more rapidly the younger the embryo, so 

 chemical growth and differentiation take place more rapidly the 



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