464 C. G. MACARTHUR AND E. A. DOISY 



the field of local dominance that is large enough to amount to a 

 slowing down temporarily of the rate of loss of growth power. 



By comparing these results with those obtained in a growth 

 series on the brain of the albino rat (W. and M, L. Koch, '13) a 

 great similarity is evident. The nature of the process, the di- 

 vision into periods, the relative amounts of the various con- 

 stituents, and their order of development are much alike. The 

 main differences are found in the larger percentage amounts of 

 lipins, with a corresponding decrease in proteins and a great 

 lengthening of the periods of growth. Thus the changes occurring 

 in the rat brain are much more rapid than in human brain, but 

 the rat brain does not attain quite the same degree of differentia- 

 tion. By comparing the data in the two series for extractives as 

 a whole, and the various extractives, no marked differences are 

 evident, indicating that the changes in rate of metabolism with 

 growth are similar in both, though the time necessary to change 

 from one corresponding physiological age to another in the rat 

 is probably but about one-thirtieth of that in the human. 



4. Nature of the' growth process 



One of the most interesting points which these data raise is that 

 of the nature of the growth process in nervous tissue. The curves 

 show that the brain as a whole, as well as each of the individual 

 substances or groups of substances (tables 12 and 13, figs. 1 and 2) 

 estimated, increases slowly in absolute amounts per unit of time 

 during the first part of development. Later the increase is 

 larger, and is then followed by a period when the amounts are 

 continually smaller. If, however, one observes the curve for the 

 amount added per unit mass of substance during a given period 

 of time (tables 14 and 15, fig. 3) it is seen that the rate of addition 

 is greatest in the youngest tissue. This rate of addition dim- 

 inishes most at first, then more slowly, and is followed by a 

 somewhat greater comparative rate of loss of growth power. 

 The first curves (absolute amounts) are smilar to those reported 

 for growth of the whole organism (Robertson, '08). Such curves 

 are by some authors supposed to indicate that the process they 



