NERVE TERMINATIONS OF THE TADPOLE. 59 
preparation it is placed in a mixture of equal parts of glycerine 
and water, with a drop or two of formic acid for every 10 cc. 
of the mixture. It is much improved by keeping in this fluid 
a month or more. The same fluid should be used in mounting 
pieces of the preparation. 
By a careful attention to the details of this method I suc- 
ceeded in obtaining in the greater number of cases very success- 
ful preparations. Acidification of the tissue with lemon juice 
or formic acid previous to treatment with gold chloride did not 
enhance the success of the results, while it often appeared to 
have a contrary effect. 
I1V.—TuHE SKIN. 
The height of the epithelial layer on the tail varies consi- 
derably in different parts of a vertical section. Near the 
middle of the lateral surface it is very often one and a half 
times what it is along the border, and between these points 
there are gradations in thickness. It sometimes happens 
that the number of layers in the epithelium may be greater 
near the middle of the lateral surface than elsewhere. The 
thickness of the epithelium varies also with the age of the 
tadpole. 
The number of layers in the epithelium of course influences 
its thickness to a certain extent. If the tadpole has been kept 
in favorable or normal conditions, the number of layers which 
can be easily distinguished is then three, often four, or five. 
Of these, the two most distinct from each other are the super- 
ficial and basal layers; while the others show stages inter- 
mediate between these two. The constituents of the basal 
layer are the largest and are cylindrical inshape. The nucleus 
is usually placed in the upper half of the cell when there are 
only two layers in the epithelium, but in the lower half of the 
. cell when there are more. The cellular contents possess few 
or no granules, and apart from the figures of Eberth are clear 
and transparent. The cells of the intermediate layers resemble 
those of the basal layer in this respect. These are usually of 
a polyhedral shape. 
