Percentage of water in the brain of the dog-fish. 39 



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The percentage of water in the brain of the dog-fish. 

 By G. G. Scott. 



[From the Department of Physiology, Columbia University, and 

 Biological Laboratory of the U. S. Bureau of Fisheries, 

 Woods Hole, Mass.] 



Donaldson ('10) has shown that in the albino rat between 

 birth and maturity the percentage of water in the brain diminishes 

 from 87.8 per cent, to 77.5 per cent. He calls attention to the 

 fact that the human brain at birth contains a greater percentage 

 of water than at maturity and from the investigations of Koch 

 and Weisbach he obtains as the percentage of water in the human 

 encephalon to be: Birth 88.3 per cent.; 2 yrs. 81. 1 per cent.; 5 

 yrs. 79.2 per cent.; 25 yrs. 77.0 per cent. He concludes that 

 probably in all mammals we will find the same range in percentage 

 of water, that the loss in water occurs in the same manner but that 

 the time required for each successive step is determined by the 

 intensity of the growth process characteristic of each period. The 

 present writer determined the percentage of water of the brains 

 of 17 spiny dog-fish (Squalus acanthias) and 97 smooth dog-fish 

 {Mustelus canis). The smooth dog-fish ranged in size from very 

 small to large. There is no such reduction in the percentage of water 

 as found by Donaldson and others in the case of the mammalian 

 brain. The average percentage of water in all the Mustelus brains 

 examined was 78.5 per cent. There was very little difference 

 between this and that obtained for the very young or the old. 

 There is an indication of a slight fall of about 2-3 per cent, between 

 birth and maturity. The great reduction (i. e., about 7 per cent.) 

 occurs in mammals during the period when the central nervous 

 system is growing most rapidly. Both the rat and man during 

 this short post-birth period pass from a helpless state to one of 

 activity. The rat is born helpless and blind, etc. The dog-fish 

 is an active free-swimming organism at birth. The present writer 

 would conclude then that the differences in reduction of water in 

 the two cases is that the nervous (and body) changes which occur 

 in the mammal are post-embryonic and extra-utero. In the dog- 

 fish they take place in utero. Determinations from brains of late 

 mbryonic stages can only settle this hypothesis. 



