Donaldson, Growth of Central Nervous System. 353 



the arc. At 105 gms. the chord and arc intersect and a hmit is 

 obtained. This point of intersection is arbitrarily chosen to indi- 

 cate that at which the rapid growth of the brain ceases. Within 

 the limits taken, the maximum deviation of the values obtained 

 by the 7th root of the body weight is 0.5 per cent, the values on 

 the logarithmic curve being considered as the standard. (Com- 

 pare table I, columns D and E, for the body weight group, 185 



rms. 



Using the formula of Dubois ('98) 



E :E' ::S-:S'- 



where E and E' are two different encephalic weights, related as a 

 given power of iS and iS', the corresponding body weights, it appears 

 that the value of x ("the exponent of relation") taken as the 7th 

 root, is in the present instance 0.143. Lapicque ('08) has en- 

 deavored to show that where individuals of the same species but 

 of different body weights are compared, we should expect the 

 value of x to be 0.25, equivalent to the 4th root of the body weight. 

 To explain why my results do not accord with those obtained by 

 Lapicque would require a long critique of his studies on this point. 

 I prefer however to leave this till another occasion, as the intro- 

 duction of it here would obscure the main point of the present 

 paper. 



To explain the essential differences between the rapid and the 

 slow growth of the brain thus indicated, it will be necessary for 

 us first to have information touching the changes in the percentage 

 of water, the chemical composition, the ether-alcohol extract, the 

 degree of medullation and the other histological modifications 

 occurring during growth, so that it is hardly worth while to dis- 

 cuss this question now. 



Before leaving the subject of the brain weight, there is still one 

 point more to be presented. It is a familiar fact that rats, even 

 of the same litter and reared together, grow very differently, and 

 therefore at the same age may have widely different body weights. 

 Moreover, either by underfeeding, or by the use of a monotonous 

 and comparatively innutritious diet, animals otherwise normal, 

 may be stunted in their growth. 



In the class first mentioned, we have designated those which 

 grew to unusual size as "giants," and those which remained small 



