THE NATURE OF GROWTH 79 



reacting with that external medium, receiving matter from the 

 same either (1) in bulk (phagocytosis), or (2) by diffusion, or (3) 

 by surface absorption, and discharging other matter into the 

 same (excretion). And if we study carefully these processes of 

 assimilation and discharge it is to be made out that the outer 

 cell membrane is semi-permeable, preventing the entrance of 

 certain substances, permitting the entrance of others, whether 

 directly, or after a preliminary dissociation of the same into 

 simpler molecules by means of its extracellular secretions. And 

 once these food-stuffs are taken into the cytoplasm, there again 

 they are, if necessary, broken down still further into yet simpler 

 molecules by intracellular enzyme action. 



It is being more and more surely established that these 

 enzymes are of nuclear origin, 1 resulting from the discharge of 

 certain constituents from the nuclear substance, but this is not 

 a necessary part of my argument. The essential point is that 

 the food-stuffs, protein, carbohydrate, and hydrocarbon, are not 

 utilized by the cell as such, but are dissociated, and it is these 

 disintegrated and dissociated molecules that are utilized by the 

 cell in one of two ways, either to supply energy for the per- 

 formance of work, or on the other hand for growth. 



THE NATURE OF GROWTH 



I feel almost apologetic for taking up time over such 

 elementary matters. And yet these are matters that, while 

 absolutely basal, are absolutely neglected by ordinary biologists. 

 Take, for example, this matter of growth. With all the extra- 

 ordinary width of his learning, D'Arcy Thompson 2 has just 

 achieved a work of some 750 octavo pages on Growth and Form, 

 full of most interesting observations and data, and with a 



1 See Pt. II. Chap. V., "The Dominance of the Nucleus." 



2 Growth and Form, Cambridge Univ. Press, 1917, p. 203. [It is but just 

 to D'Arcy Thompson to point out that his title was meant by him to signify 

 and that by a figure of speech which, while wholly permissible, may 

 be construed otherwise " Form in relationship to growth " ; in other 

 words, while he occasionally touches upon matters physiological the work is 

 essentially morphological, " an easy introduction to the study of organic Form," 

 as he terms it in his Preface. Perhaps I may be forgiven if it was just these 

 occasional physiological excursions that arrested my attention rather than 

 the more severely mathematic and physical groundwork of his volume.] 



