434 Chemical Differentiation of the Brain 



are combined with such lipoids as the sulphatides and kephalines (a sub- 

 group of phosphatides), while the Ca and Mg have more tendency to remain 

 combined with the proteins. 



Anatomical distribution and physiological significance. Very little is 

 known of the anatomical distribution of the salts except as shown by the 

 work of Macallum 25 which demonstrates that chlorides and potassium are 

 associated with the nerve fiber. According to Alcock, 26 potassium is sup- 

 posed to play an important role in the propagation of the nerve impulse. 



With this introductory statement of the anatomical distribu- 

 tion and physiological significance of the substances quantitatively 

 determined, we may now present the results of a study of their 

 variation during the growth of the brain. 



The brain of the albino rat was selected for this study, for the 

 reasons already presented in the first paper of this series. 27 



From a comparison of the brain of the albino rat at birth and 

 the brain of the fetal pig, it was found that the brain of the new 

 born rat is as young nervous material as can conveniently be 

 analyzed at present. It forms therefore a suitable starting point 

 for this study of chemical differentiation during growth. The 

 analyses reported in this paper are those of the brains of rats 

 aged respectively, 1, 10, 20, 40, 120, and 210 days. The results 

 show that it was possible to follow closely the various structural 

 changes which occur during the differentiation of the growing 

 nervous system 



The material was furnished by the Wistar Institute of Anatomy; 

 the brains being collected and analyzed in the manner already 

 detailed. 28 Koch's quantitative methods were used. 29 



RESULTS OF ANALYSES. 



The results of analyses are embodied in Table II. Duplicate 

 analyses have been carried on throughout, and are summarized 

 in Table III. This table gives the averages of the analyses, except 



25 Macallum, A. B. : Journ. ofPhysioL, xxxii, pp. 95-128, 1905; Macallum, 

 A. B. and Menten, M. L. : Report 75th Meeting British Assoc. Adv. Sti., 

 p. 555, 1906. 



26 Alcock, N. H.: Journ. of Physiol., xxxix, pp. 402-410, 1911. 



27 Koch, Mathilde L. : this Journal, xiv, pp. 267-279, 1913. 



28 Koch, Mathilde L.: loc. cit. 



29 Koch, W.: Journ. of Amer. Chem. Soc., xxxi, pp. 1335-1364, 1909. 



