276 Chemical Differentiation of the Brain 



The first appearance of the cells in the outer granular layer of the 

 cerebellum in the rat is in the 19-day fetus, and in the .pig in the 

 50 mm. pig fetus. At this time the cells are settled in a thin layer 

 (two rows deep) around the outer edge of the cerebellar cortex. 

 This layer increases until a considerable depth is filled in with 

 cells which soon separate into two strata, an outer and an inner; 

 this separation takes place in the rat at birth, and in the pig fetus 

 when it is 100 mm. in length. The cells from the outer granular 

 layer now begin to migrate to the inner granular layer and the dis- 

 appearance of the cells from this outer granular layer, which corres- 

 ponds with the time of securing motor control in an animal, occurs 

 in the rat at the twenty-first day of life, and in the pig when this 

 is from 200 to 300 mm. in length, or at birth. These facts show, 

 therefore, that the new born rat is as developed with respect to 

 motor activity as the 100 mm. pig fetus, and that the rat at wean- 

 ing (17-21 days after birth) and the pig at birth are at correspond- 

 ing physiological ages. The conclusion from this anatomical com- 

 parison, namely, that the 100 mm. pig fetus and the rat at birth 

 are of like physiological age, fully confirms, therefore, the conclu- 

 sion drawn from both chemical and physiological evidence already 

 adduced. 



We now ask the question, how far this result, that the nervous 

 system of the new-born rat is chemically as old as that of the 

 100 mm. pig fetus, agrees with observations made by Donaldson, 

 that the rate of growth and percentage of water in the mammalian 

 nervous system (represented by the brains of man and the rat) 

 agree in the two forms at equivalent ages, thus indicating that the 

 nervous systems are in corresponding physiological states at equal 

 fractions of the life cycles. 16 



18 H. H. Donaldson: A Comparison of the White Rat with Man in respect 

 to the Growth of the Entire Body, Boas Anniversary Volume, 1906, pp. 

 5-26. 



