32-t William E. Kellicott 



cm., or, including both sexes, about 1425 grams, 81 cm. : probably these 

 figures are fairh' close to the actual averages. It is to be regretted that 

 the exact ages of these fish can not be told. I have been unable to find 

 any statement regarding the time rate of growth of any of the Elasmo- 

 branchs. While we may nevertheless continue to use the expression 

 "rate of growth" in speaking of the brain and viscera, it must be remem- 

 bered that not the time rate but the comparative rate of growth is meant, 

 the weight of the entire organism serving as the basis of comparison. 



The Data 



The Brain. — At birth the average weight of the brain is found to be 

 0.855 gram. After birth, as shown in Plate 1, its weight increases 

 rapidl}', but at a slightly diminishing rate, so that in fish of about average 

 (not middle) size the brain weighs approximately 3.5 grams. Among the 

 laro-er individuals the diminution is much slower but is continued, though 

 the growth of the brain does not cease during life. The heaviest brain 

 weighed 7.2 grams. While the total weight had increased over 110 times 

 the brain had increased but 8.4 times. The curve shows that this heaviest 

 brain was somewhat larger than the expected weight, which would l)e 

 only about 6.4 grams, giving an increase of 7.5 times. 



The curve showing increase in absolute weight of the brain is, how- 

 ever, less significant than that of its relative weight: this also is shown 

 in Plate 1. At birth the ratio of brain weight to body weight is very 

 high — 1.116 per cent. But this falls very rapidly while the animals 

 are increasing up to 300 or 400 grams. Then gradually the ratio de- 

 creases until in fish of average size it is only 0.25 per cent. This decrease 

 continues at a diminished rate throughout life, so that it is still falling 

 in the largest specimens examined: in the largest it was but 0.085 per 

 cent. In other words the brain of this largest fish was, compared to 

 the total weight, only one-thirteenth as large as in those just born. 



It is not possible to distinguish between the sexes with respect to 

 brain weight. The heaviest male examined weighed 3010.5 gi-ams, or 

 considerably less than one-half the heaviest female, but the weights 

 of the brain, both absolute and relative, in males and females of the 

 same total weight were not sensibly different, as can be seen from inspec- 

 tion of the plates where the sexes are charted distinctively. The curves 

 given by the brain weights are remarkably smooth and the individuals 

 grouped closely about the curve. 



