552 ON INCREASE IN COMPLEXITY [pt. iii 



former is descending and the latter ascending, they are doing it in 

 the same phase, i.e. slowly at first and more quickly afterwards. 

 Probably the most significant result which emerges from Murray's 

 symmetrically diphasic plan is that growth-rate and differentiation- 

 rate go together, and in opposition to metabolic rate. 



Enriquez's paper of 1909 contains a foreshadowing of the idea of 

 chemical differentiation (see also Scammon & Ness). 



S'S. Chemical Processes and Organic Form 



In considering the processes of differentiation in the embryo, there 

 has been much disinclination to admit their physico-chemical 

 nature. It is the great credit of His that he took the lead in this 

 direction, pointing out that processes such as the production of the 

 neural and amniotic folds were the inevitable results of unequal 

 growth controlled by physico-chemical factors taking place in what 

 was, to start with, an undifferentiated sheet of embryonic cells. 

 His's artificial blastoderms, again, made of pills of dough to which 

 varying amounts of yeast in various places are added, provide a 

 close model for the embryo. D'Arcy Thompson has described the 

 incredulity and opposition which the views of His met with, especially 

 after the publication of his classical letter in the Proceedings of the 

 Royal Society of Edinburgh in 1888. As Garbovski put it, "it is absurd 

 to treat the living being as if it were made up of vesicles, cylinders, 

 and plates, and not of vital units". Embryologists such as Hertwig 

 and Balfour held that, in the study of development, a sufficient causal 

 explanation of one stage had been given when the immediately 

 preceding stage had been adequately described. " My own attempts", 

 said His in a famous passage, "to introduce some elementary 

 mechanical or physiological conceptions into embryology have not 

 been generally agreed to by morphologists. To one it seemed ridiculous 

 to speak of the elasticity of the germinal layers; another thought 

 that by such considerations we put the cart before the horse; and 

 one more recent author states that we have better things to do in 

 embryology than to discuss tensions of germinal layers and similar 

 questions, since all embryological explanations must necessarily be of 

 a phylogenetic nature. This opposition to the application of the 

 fundamental principles of science to embryological questions would 

 hardly be intelligible if it had not a dogmatic background. No other 

 explanation of living forms is allowed than heredity and any which 



