128 Francis B. Sumner 
therefore an actual difference in the volume of the tail. The 
number of caudal vertebrz was likewise counted for all of this 
Fig. 7 Series of 1907-1908: ear length of Fig. 8 Series of 1907-1908: foot length of 
LP) 
““unmated males” at 24 months of age. “Cunmated males” at 24 months of age. 
group. Ihe mean figures are given in the table* (6), where it 
is seen that the difference between the two groups (if significant 
5 The number of vertebra was counted, from the sacrum to the last nodule of bone distinguishable 
under a low power objective. The task was comparatively simple and admitted of tolerable accuracy. 
The average error of observation was probably less than one vertebra per mouse. Since the skele- 
