Mathilde L. Koch 279 



SUMMARY. 



1. A quantitative determination of the constituents of the brain 

 of the albino rat at birth shows it to be chemically as undifferenti- 

 ated as the brain of a 50 mm. or 100 mm. pig fetus. 



2. There is little difference in chemical composition between 

 the 50 mm. and the 100 mm. pig fetus brain. 



3. Since the 50 mm. fetus brain is the youngest which can be 

 analyzed and this closely resembles the 100 mm. fetus, and this 

 in turn is no more immature than the new born rat, it appears that 

 the brain of the new born rat is sufficiently immature to serve as 

 a starting point in a study of the chemical differentiation -of the 

 brain during growth. 



4. That the brain of the new born rat is as immature as the 

 100 mm. pig embryo is shown, also, by the similarity of the changes 

 in the outer layer of cells of the cerebellar cortex in both animals 

 previous to gaining motor control, and by the animal's behavior 

 at this period. 



5. If the nervous systems are assumed to be in corresponding 

 states when motor control is obtained, and Donaldson's law is 

 correct, that the nervous system is in the same state at correspond- 

 ing physiological ages, then the brain of the rat at birth should 

 correspond chemically with the 100 mm. pig fetus brain. This is 

 found to be the case. 



