102 Francs B. Sumner 
of each mouse were used, and it was sought to take them from 
corresponding regions of the skin in the case of every animal. 
The entire process required much time, and the counting of the 
hairs proved to be extremely fatiguing to the eyes. Moreover 
the great individual differences in the density of the hair and even 
the differences in density on the two sides of the same individual 
showed that satisfactory figures could not be obtained without 
making a large number of determinations. For these and for 
other reasons the counting of hairs was abandoned after the first 
year’s series of experiments. ‘The results, so far as obtained, 
will be presented later in the paper. 
The method of weighing the total pelage of each mouse has 
proved to be by far the most satisfactory one. Aside from the 
greater ease and accuracy of technique, this method has the advan- 
tage of showing whether or not the total quantity of hair has varied 
under the different conditions. It has the disadvantage, however, 
of not showing how the quantity has been affected, 1. e., whether 
by the augmentation of the number of developed hairs, or of their 
length or their diameter. The process employed was as follows :° 
The pelt after drying was placed in a saturated solution of cal- 
cium hydrate, and left for three days (occasionally longer).*° After 
rinsing, the hair was scraped off with a moderately sharp scalpel. 
Most of the hair, in the majority of pelts, could be removed very 
readily; the remainder sometimes required considerable scraping. 
The epidermal cells were unavoidably scraped off at the same 
time, but these furnished very little material, and that was for 
the most part removed by washing. In any case their presence 
constituted a source of error which was probably practically con- 
stant throughout the series. ‘The hair, after a thorough washing 
in water, was treated for ten to twenty minutes with dilute HCl 
(5 per cent) in order to remove any calcium which might remain 
in association with it. After further repeated washings, it was 
dehydrated in 95 per cent alcohol, subjected to ether for one hour, 
® Acknowledgment is due to Dr. C. L. Alsberg, of the Bureau of Plant Industry, for valuable sug- 
gestions relating to certain steps in this process. 
10 An equal number of skins of the two lots were always treated at the same time, so that constant 
differences in treatment were avoided. 
