132 HENRY H. DONALDSON 



The significance of table 5 is made more evident by plotting 

 the data on a base line giving either the weight of the brain or 

 of the spinal cord. It is then seen that the records for the weight 

 of water lie in an approximately straight line. 



Weight of water in the brain. Beginning with the brain, chart 

 4, it is seen that when the lines representing the actual weight 

 of water are contrasted with the dotted line, showing the amount 

 of water necessary to maintain the percentage constant at its 

 initial value, the former ascend less rapidly. Further inspection 

 shows that the lines representing the increments of water as 

 observed are slightly convex. This is true for both sexes. We 

 will first consider in detail the relations as thus shown for the 

 males. 



A straight line drawn between the terminals for the male curve 

 corresponds to an average of 73.6 per cent of water in the incre- 

 ments of weight after a brain weight of 0.35 grams. Since, however, 

 the curve is slightly convex, it is better represented by two straight 

 lines, one drawn from the initial entry to the entry above the 

 brain weight of 1.05 grams, and the second from this latter to 

 the final entry at 2.15 grains. The angle of the former line 

 corresponds to 76.4 per cent, of water and that of the latter to 

 71.8 per cent. 



From this it appears that the earlier increments of brain weight 

 have a somewhat greater percentage of water than those acquired 

 later. 



It is to be noted however that the earlier period comes to an 

 end when the animal weighs only 17 grams, and is about 15 days 

 old (see chart 4) although by this age the very rapid growth of 

 the brain in weight has been completed. (See Donaldson '08, 

 plate III, chart 3.) 



With slight differences, which are not significant, the relations 

 here described for the males hold for the females also, but it is 

 hardly necessary to give the determinations in detail. 



Such are the general relations of the increase in the weight of 

 water with the increase in brain weight. By these relations 

 several facts are shown. 



First. The proportion of water in the brain diminishes with 



