424 Chemical Differentiation of the Brain 



The selection of chemical methods for such a study was largely 

 guided by the principle now coming to be generally accepted, 

 namely, that in living matter we are not dealing with an aggre- 

 gation of more or less similar, highly organized and necessarily 

 complex molecules (Riesenmolekul of Pfltiger), but rather, with 

 a more or less heterogeneous substratum in which dissimilar and 

 not necessarily highly complex molecules, or their dissociated par- 

 ticles, are engaged in a series of correlated chemical reactions. 

 The larger aggregates may be conceived as either not taking 

 part directly in chemical activity, or as helping in the control 

 and localization of the chemical reactions, just as in a photo- 

 graphic dry plate, the presence of the gelatin makes possible a 

 high degree of localization of the photo-chemical reaction. We 

 aimed, therefore, to stop the chemical activities at definite given 

 stages during the growth period and then to observe the differ- 

 ences which could be demonstrated. From such data we then 

 drew conclusions as to the nature of the transformations which 

 had occurred in the interval. 



The methods of collecting the material were devised with this 

 end in view, namely, to stop all chemical activity as rapidly 

 and completely as possible. The sources of error due to post 

 mortem changes then became constant, and we are in reality 

 following a principle that has long been in use in histological 

 studies. For the preserving agent, alcohol was selected, as it is 

 the least apt to interfere with the further chemical procedure, 

 and, in fact, treatment with alcohol represents a step in the 

 process. 



In the selection of the chemical methods for this series two 

 points were kept in mind: 



1. The necessity of correlating the chemical observations with 

 the known facts of structure, to the interpretation of which they 

 should add a greater precision. As an example of this, there 

 were studied the sulphatides (lipoid sulphur) which are intimately 

 associated in the nervous system with the sheaths of the medul- 

 lated nerve fibers. 



2. The collection of data, which, correlated with function, 

 would give the physiologist a better knowledge of the nature of 

 his material and thus enable him to do more than speculate as 

 to the probable nature of the processes involved in the phenomena 



