COMPOSITION OF BRAIN OP 1 ATAXIC PIGEONS 107 



brum would seem to be the least differentiated, or youngest, of 

 these 'brain tissues.' Not quite all of the figures agree in the 

 assignment of a particular brain type to its place in the series. 

 Certainly, however, there is an interesting agreement. 



Do these figures have any phylogenetic meaning? Does the 

 known sequence of chemical differentiation in ontogeny have 

 any relation to phylogenetic facts? As the types of brains stand 

 in the series, the human cerebrum shows the highest chemical 

 differentiation; the pigeon cerebrum is the least differentiated. 

 The cerebella occupy intermediate positions, that of the pigeon 

 being lower than the human. 16 



Of course, differences in proportion of white and gray sub- 

 stance are involved, but possibly neurologists may have at hand, 

 or may later note, other facts which have a relation to these com- 

 parisons of chemical composition and to this grouping of chemical 

 types of brain on the basis of age. It has already been shown by 

 Donaldson ('08, '10) that two main phases of brain growth in 

 man and the rat are similar at corresponding ages and that the 

 percentage of water in the brain agrees at equivalent ages. 

 Hatai ('17) concluded "that the percentage of water (in body of 

 different mammals) is an indicator of the chemical alteration in 

 different species, while neither the calendar age nor body weight 

 of the animals can be used for this purpose." 



16 It should be shown that the relative position of these brain parts is not a 

 fortuitous result of the particular ages of the human and pigeon brains selected 

 for comparison. The human brains were aged 20 and 54 years; the pigeons were 

 of 205 and 598 days. These pigeons are sexually mature at 180 days. Five 

 hundred and ninety-eight days is more than three times the period preceding 

 sexual maturity. By this method of computing age, the two groups seem compa- 

 rable. If, moreover, the figures for either the 205-day or 598-day birds be taken to 

 represent properly the composition of the pigeon brain, none of the figures of the 

 two parts of table 9 are changed or misplaced in relation to the other figures. If 

 the 20-year human alone be made to serve as a basis of comparison, the order of 

 none of the figures is changed. If the 54-year human be made to represent the 

 human, then the only changes of order concern the moisture of the human cere- 

 bellum-medulla which falls slightly below (initially it is only 0.2 per cent above) 

 that of the pigeon cerebellum-medulla, and thus makes the series more perfect 

 for the water fraction than it stands in the table. One slight additional change 

 results: The extractives of the human cerebellum-medulla fall very slightly 

 below the extractives of the human cerebrum. 



