550 ON INCREASE IN COMPLEXITY [pt. iii 



fundamental processes may be distinguished, falling into two groups. 

 The first group consists of those processes which have their maximum 

 rate of change early in development, and it includes both growth- 

 rate and differentiation-rate. The second group consists of those 

 processes which have their maximum rate of change late in develop- 

 ment, and it includes, firstly, the concentration of solids in the 

 embryo (called by Murray "chemical growth ") , secondly, the increase 

 in complexity or change of composition in the solid substance (called 

 by Murray "chemical differentiation") *, and thirdly, the metabolic 

 rate or respiratory intensity. These two groups of processes correspond 

 to two type curves which in some circumstances show skew symmetry 

 round a central point. The first group has been termed by Murray 

 the group of "primary integration" and the second one the group 

 of "secondary integration", and he has in various papers associated 

 the two groups with the primary and secondary redistributions of the 

 evolutionary process in Herbert Spencer's scheme, or, as they are 

 often called, simple and compound evolution. 



Murray found that the surface volume ratio was also a member 

 of the first group, changing most rapidly in the earliest stages and 

 falling in a way not unlike the growth-rate. He calculated it from 



the formula of Meeh : 2 



S= KW\ 



and, although such an application to the chick embryo rests in- 

 evitably on several unproved assumptions, it is probable that his 

 curve is not wholly misleading (see Fig. 94) . More interesting still, 

 he found in a later paper that the absorption curve (the grams 

 absorbed per gram dry weight per day) was not a member of either 

 group, for it was sigmoid, falling rapidly for a time, then less rapidly, 

 and then more rapidly again. His absorption curve, however, was 

 derived from the oxygen measurements which have already been 

 criticised, a fact which might account for its S-shaped character. 

 About the same time, I also calculated the curve of absorption-rate, 

 not from respiration experiments but from direct measurements of 

 protein, fat and carbohydrate, and I obtained a curve belonging to 

 group I, i.e. changing most rapidly at the beginning of development. 

 It is very interesting that the curve of absorption-rate should be 

 found to follow the curve for surface/volume ratio (see Fig. 253). 



* This process of "chemical differentiation" must not be confused with "chemo- 

 differentiation " (seep. 571). 



