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



early fetal life, but continues to increase in absolute amounts 

 (table 12) until maturity, when there is about twice as much as 

 of nucleoprotein a and half as much as of neurokeratin (Mc- 

 Gregor, '17). Nucleoprotein a possibly is always present, but 

 probably is largest in percentage amounts when nerve growth 

 and activity are greatest. It would not do even to guess how 

 these last two proteins are distributed in the brain. 



The total protein curve (fig. 1) indicates that some particular 

 protein (possibly nucleoprotein b) is an especially important 

 factor in the subsequent growth of the brain. It seems to lead 

 in the increases that take place. 



Extractives 



The separation of extractives into organic and inorganic, as 

 given in the data, is of but little value because of the fact that 

 such a separation, based on the solubility of organic constituents 

 in alcohol and the insolubility of the inorganic ones in alcohol (or 

 on the residue after ignition), is very unreliable. The data given 

 are merely suggestive. Howev-er, the determination of total ex- 

 tractives is rather accurate. Inosit, urea, leucin, tyrosin, taurin, 

 hjTDOxanthin, and peptones are a few of the organic substances 

 present in this fraction. In general it may be stated that the 

 larger the percentage of these simpler crystalloidal molecules, 

 the more rapid the metabolism and the younger the tissue. 

 Various inorganic salts of sodium, potassium, ammonia, calcium, 

 magnesia, and iron have about the same significance. While the 

 rate of growth is high, these constituents are present in larger 

 percentage amounts (table 11), but with a decrease in rate of 

 development they rapidly decrease in rate of formation, until 

 after two years of age they are but very slowly increased in ab- 

 solute amount (table 12). 



They are present in larger amounts in cells than in axis cyl- 

 inders. Potassium salts and chlorides are supposed to be re- 

 lated to nerve conductivity (Alcock and Lynch, '11). 



In drawing conclusions concerning the rate of activity of nerve 

 tissue from the percentage amounts of extractives, one needs to 



