Mathilda L. Koch 273 



nervous tissue during growth and may therefore be used in making 

 a comparison between the brain of the new born rat and that of the 

 fetal pig to determine which is the more immature. 



We may now proceed to consider in detail the comparison of the 

 various constituents 8 in the brain of the new born rat and the pig 

 fetus. 



Water. The per cent of water in the brain of the new born 

 rat is closely similar to, but a little lower than, that of either the 

 50 mm. or 100 mm. pig fetus. This would indicate that the rat is 

 of about the same physiological age as these fetuses, since the 

 differences are within the limits of error. 



Protein. The per cent of protein in the total solids is higher in 

 the brain of the new born rat than in either that of the 50 or the 

 100 mm. pig fetuses. Since the per cent of protein is highest in 

 the youngest material, this is an indication that the rat's brain is 

 less mature than that of the 100 mm. pig fetus, but not very 

 different from the 50 mm. fetus. 



Phosphatides. The per cent of phosphatides is the same in the 

 new born rat as it is in the 50 and the 100 mm. pig fetus. This 

 would indicate a close agreement in physiological age between 

 these two. This is the lowest phosphatide content yet obtained in 

 an analysis of the brain tissue and approaches that observed in the 

 suprarenal, which, among all the organs, comes closest to that of 

 nervous tissue in chemical composition. 



Cerebrosides. These are absent in both the new born rat, and 

 in the pig fetus, as is to be expected in nervous tissue before medul- 

 lation. 



Sulphatides. The percentage of sulphatides is about the same 

 in the new born rat as in the pig fetus, which indicates the same 

 age. 



Organic extractives and inorganic constituents. These are some- 

 what higher in the pig fetus than in the new born rat and, except as 

 this is associated with the greater per cent of lymph in the embry- 

 onic material, it would indicate it to be more immature than the 

 new born rat. 



8 The nature and significance of the constituents will be discussed in the 

 third paper of this series. 



