CHEMICAL CHANGES IN HUMAN BRAIN 467 



not divide so often when there are more of them in a more un- 

 favorable medium is that the individual cells do not grow to the 

 dividing stage so quickly. However, one can apply the geomet- 

 rical ratio idea to the development of the individual cell if that 

 is found to increase in absolute weight fastest during the middle 

 period of growth. In fact, it is rather to be expected that such 

 would be the form of its growth, without its being in any way 

 related to autocatalysis. For if the protoplasm formed on cell 

 division is thought of as a unit of protoplasm, it would form two 

 units in a certain period of time; then these two would form four, 

 and so on through the geometrical series, if no retarding factors 

 were present. But there are undoubtedly such factors, so we get 

 essentially the autocatalytic phenomenon. The size of the cell, 

 the relation of size of the nucleus to that of the cytoplasm, the 

 amount of cell differentiation, complexity of colloidal substratum 

 of cell are large factors in determining this form of growth. There 

 seems to be a physiological state that is rather definite for any 

 kind of cell, which, unaltered, tends to make the cells increase. 

 One sees the necessity, granting the power of protoplasm to pro- 

 duce more material like itself, in an increasingly unfavorable 

 environment, for the S-shaped curve of growth. This is inde- 

 pendent of the question why growth takes place; it is true, irre- 

 spective of the nature of the growth impulse. It is probably not 

 wise, however, to speak of such growth as autocatalytic, because 

 it probably does not have a chemical autocatalytic basis. Though 

 enzymes seem to play a part in it, it is not necessarily enzymic at 

 all, much less autocatalytic and monomolecular. Probably any- 

 thing that can increase geometrically, put under progressively 

 less favorable conditions, whether living or not (say, the growth 

 of a crystal in a slightly supersaturated solution), would give an 

 autocatalytic form of increase. 



SUMMARY 



1. During growth the proteins, phosphatids, sulphatids, cere- 

 brosides, cholesterol, and total solids increase in percentage 

 amounts. There is but slight change in the percentage of ex- 



