520 Journal of Comparative Neurology and Psychology. 



their food.' If one takes into account the great strain of suck- 

 ling ten or twelve young for the period of twenty days, one 

 can easily understand that some such provision as this is neces- 

 sary. This surplus in weight is largely lost during the time the 

 mother is suckling her young. 



Observations upon the Nervous System of the White Rat. 



As soon as the observations on the body growth were com- 

 pleted, the rats were killed, and the separate weights of the 

 brain and spinal cord, together with the percentage of water in 

 each of these parts were determined. The cord was severed 

 from the encephalon at the tip of the calatmis scriptoriiis. The 

 spinal roots were cut as close to the cord as possible. 



After the weight of the fresh brain and cord was found, 

 the percentage of water present in each was determined. The 

 Iresh brains and cords were put in an oven and subjected for 

 eight days to a constant temperature of 90° Fahrenheit. They 

 were then taken from the oven and cooled for thirty minutes 

 in a sulphuric acid drying-chamber. The weight of the dried 

 brains and cords was found, and with this completion of the 

 data, the percentage of the water present in each was determin- 

 ed. 



Table II gives the final results for both the mated and the 

 unmated series of rats, on the weight of the body, the weight 

 of the encephalon and cord, and the percentage of water present 

 in each. 



reverse is true in the case of the rat. The older the young of the rat up to 25-30 

 days, the greater the demand upon the mother's milk supply and the consequent 

 increasing exhaustion of the mother. If the litter is large, the weight of the 

 mother is sometimes reduced to a weight which is below that of the corresponding 

 unmated rats. The explanation is, of course, that the young guinea pigs are rel- 

 atively large when born and become independent of the mother much earlier 

 than the young rats. 



'MiNOT. See reference above, p. 145. 



