Mathilde L. Koch 275 



immature chemically than that of the new born rat, it appears 

 that the new born rat's brain is as young nervous material as can 

 conveniently be analyzed at present: and it forms, therefore, 

 a convenient starting point for the study of the chemical differen- 

 tiation of the central nervous system during growth. 



CHEMICAL RESULTS COMFIRMED BY PHYSIOLOGICAL AND ANATOMICAL 



DATA. 



It is astonishing that chemically the brain of the new born rat 

 should be as immature as that of the 100 mm. pig fetus, but, 

 surprising as this fact is, it is substantiated by a comparison of 

 the structure of the cerebellum of these two animals and of their 

 behavior at the time of birth. 



It is a well-known fact that the rat is born in a very immature 

 state, with its eyes shut, and when first born, is capable only of 

 movements involved in sucking, bending the body and tail and 

 making a squeaking noise. 12 The pig, on the other hand, "is born 

 with its eyes open and requires no assistance as a rule in making its 

 start in life. It is more or less able to walk around as soon as 

 born." 13 Such a state of activity in the rat is not reached until 

 the period of weaning 17-21 days after birth. 



This difference in physiological behavior is correlated with the 

 relative development of the cerebellum of the two animals; par- 

 ticularly as indicated by the development and transformation of 

 the outer granular layer of cells. A comparison of this layer in 

 both animals, founded on the observations of Addison 14 who 

 studied the different layers of the cerebellum in the albino rat, 

 and of Takasu 15 who studied these same layers in the pig fetus, 

 brought out the following facts : 



12 Wm. H. F. Addison: The Development of the Purkinje Cells and the 

 Cortical Layers in the Cerebellum of the Albino Rat, Journ. of Comp.N enrol., 

 xxi, p. 476, 1911. 



13 Forbes: personal communication. 



14 Wm. H. F. Addison: Journ. ofComp. Neural., xxi, p. 464, 1911. 



15 K. Takasu: Zur Entwicklung der Ganglienzellen der Kleinhirnrinde 

 des Schweines, Anat. Anz., xxvi, pp. 225-32, 1905. 



