50 SHINKISHI HATAI 
the same age, a wide range of variation might exist in respect 
to the degree of myelination, according to the environment of 
the fish or to the general nutritional conditions. Whether or 
not this suggestion has a value, only further investigation can 
determine. 
Scott (12) found that the percentage of water in the brain of 
the smooth dogfish differs very little between small and large 
specimens, and gives on the average 78.5 per cent. Donaldson 
(05) who examined the brains of the summer flounder (Para- 
lichthys dentatus) noted also but slight variation in the percent- 
age of water in the brains of large and small individuals. The 
average from sixteen flounders in which the body weight ranges 
from 539 grams to 1290 grams, is 78.45 per cent. Thus the 
average percentages of water obtained by Donaldson, Scott and 
by myself are 78.45 per cent (flounder), 78.5 per cent (dogfish) 
and 78.61 per cent (gray snapper) respectively. For the purpose 
of comparison I gave the percentages of water in the brain of 
several fish, as determined by various investigators. 
As will be seen from table 4 despite the widely different sizes 
and probably wide differences in the age of fish, the percentages 
of water in the brains are very close to one another, and further 
interest lies in the fact that the values given by the fish brains 
are not much different from the percentage of water in the adult 
mammalian brain. 
Since the reduction in the water in the brain is induced by the 
deposition of the so called ‘myelin substance’ (Donaldson, ’16) 
we may infer that the process of myelination in the fish brain 
attains its mature form at a very early period’ thus permitting 
but very slight variation from small to large individuals. Scott 
(12) also concludes from his observations on the water content 
1 In a private communication Dr. G. W. Bartelmez informs me that in Ameiu- 
rus melas, larvae 10 to 12 mm. long show already well advanced myelination of 
the roots of all the cranial nerves, as well as of the fasciculus longitudinalis medialis. 
The age of the larvae, according to Dr. Bartelmez’s estimate, is about ten to 
twelve days after fertilization. The largest adults measure as much as 120 mm. 
or nearly ten times the length of the larvae in which the myelination is already 
well advanced. From the above we may safely assume that myelination takes 
place in the fish at an early stage of development. 
