CHEMICAL CHANGES IN HUMAN BKAIN 465 



represent is an autocatalytic one (a chemical reaction that in- 

 creases in speed at first because of the catalj^zing effect of a 

 product of the reaction, and then slows down, because of the re- 

 tarding effect of larger amounts of a product of the reaction and 

 decrease in the original substance). Can the second curve, 

 however, be reconciled with this theory? From this curve it 

 would seem that the rate of reaction is fastest at first and slows 

 down continuously during growth (Meyer, '14). 



There is no inconsistency between these two facts (1st, that the 

 absolute amount of substance added is greatest during the middle 

 period of development; 2d, that the amount of substance added 

 per unit mass is greatest at an early period of development) if 

 we make certain assumptions. It is necessary to assume that all 

 or nearly all of the substance (or group of substances, or total 

 brain, or total organism) is a product of this reaction or determined 

 by some other reaction. The substances .weighed are entirely 

 (within limits of error in data) the product of something not 

 weighed or too small to make a significant difference in the 

 weighing. This means that the cytoplasm, and probably nucleus 

 (Loeb, '06), is a product of something else either present and very 

 small, or absent, or not weighable. The easiest interpretation 

 of this difficulty is to invoke the aid of vitalism. This would 

 furnish our unweighable element that determines the growth of 

 even the protoplasm. However, if something a little more sub- 

 stantial is required, one can assume the presence in the brain 

 (or in some other part of the body connected physiologically 

 with the brain) of a very small amount of a substance that in 

 some way determines the formation of all other substances in the 

 tissue (or organism) considered. This substance probably would 

 decrease during growth. One or more of its products would 

 catalyze its effect on formation of other substances. It might 

 exert its control over other reactions by operating over a longer 

 period of time or by having an unusual nature. It is conceivable 

 that a hormone or enzyme-like compound might have such un- 

 limited power. This would assume that at fertilization, or soon 

 after, this substance was made, and that subsequent develop- 

 ment is essentially a product of it. Aging would mean the using 



