Donaldson, Nervous System of the Frog, 323 



VI. Explanation of Chart I. 



This chart exhibits the weight and length of the brain and 

 spinal cord in frogs of different body-weights. 



In constructing the curves, the unit of length which on the 

 axis of abscissas represents one gram of body weight, on the 

 axis of ordinates represents one milligram of brain or spinal 

 cord weight. Two and a half of these units represent one mil- 

 limeter in the curves illustrating the length of the brain and 

 spinal cord. The chart is based on 18 cases taken from Table 

 7. The curves depend on single observations, and hence are 

 liable to exhibit irregularities. The cases chosen were in- 

 tended to give a curve falling within the extreme records. 



By this method of selection the curve for the weight of the 

 brain is roost affected. It appears to be still rising, but Table 7 

 shows that the extreme weight indicated in the curve has been 

 slightly surpassed twice by frogs weighing less than 250 grams. 



On the other hand the largest frog obtained weighed 411 

 grams. The central nervous system had been left in the body 

 twenty-four hours after death and therefore attained an abnor- 

 mal weight which excluded it from the records. After the 

 twenty-four hours in a refrigerator, the brain weight was . 340 

 grams ; the spinal cord weight was . 168 grams. 



Assuming that the brain weight is 50 % above the normal, 

 and that the cord weight is 40 %, the corrected figures 

 would be : 



Brain weight, .226 grams. 



Cord weight, .120 grams. 



On comparing these results with the last entries in Chart I, 

 they suggest that there is still some opportunity for the brain 

 to increase in weight slightly and the cord rather more and both 

 of these suggestions are plausible. 



VII. Method of Observation. 



I have endeavored to examine a series of Bull-frogs 

 from small to large. Very large frogs were not obtaina- 

 able. Some were examined which were larger than those re. 

 corded, but in every case they had been the victims of broken 



