90 MATHILDE L. KOCH AND OSCAR RIDDLE 



stood from a study of the data of table 4 and partly from the data 

 previously obtained by us ('18, table 3; in part reproduced here, 

 table 8) . It is clear that neither females of 42 days nor males of 

 51 days (averages on table 4) have fully developed brains. Two 

 females of 69 and 127 days ('18, table 3), however, each had a 

 brain nearly as large (1.811 grams and 1.813 grams) as that of 

 the largest female brain of groups I and II (K235 = 1.839 grams, 

 age 262 days, table 1) and larger than the average brain (1.803 

 grams and 1.797 grams, table 4) of the females of these much 

 older normal groups. Similarly, a male of 124 days ('18, table 3) 

 had a brain larger (1.943 grams) than the average (1.928 grams) 

 of eight normal males of 645 days (average, table 4). 



It is reasonably clear that in this particular strain of birds the 

 maximum brain weight is usually attained not much later than 

 100 days after the beginning of development (eighteen days for 

 incubation). In all of our present and previous analyses of 

 pigeon brains (table 8), therefore, only the brains of group VI 

 of the present series were undersized because of age. Groups II 

 and IV, which are compared with each other, have each two birds 

 aged less than 100 days. 



Relation of ataxia to brain size 



The relation of brain size to normality and ataxia may now be 

 confidently studied, since the influence of sex, body weight, and 

 age have already been considered. Four quite comparable groups 

 (I to IV, table 4) are available ; two of these are brains from nor- 

 mal birds and two from ataxics, and there are both males and 

 females in each of the four groups for comparison. The following 

 is found: 



The whole brain of each of the ataxic groups is smaller 6 than 

 that of either of the two normal groups (tables 1 and 2). The 



6 There is a high percentage of males in one normal group and a high percent- 

 age of females in one ataxic group which considerably affects the brain size of 

 these two. But the comparison between the normal and ataxic males of these 

 two groups, and between the normal and ataxic females of these two groups, is 

 just as val.d as are the similar comparisons between the other two groups in which 

 no disparity of sex exists. The mean weights of the various groups permit a 

 quite fair comparison from one group to another. 



