72 THE FROG 
F. The Nervous System.—Dissections of the central nervous 
system made by a beginner are seldom satisfactory, hence it is im- 
portant that a supply of perfect preparations be provided. In 
particular see that the roots of the spinal and cranial nerves are 
intact. For these preparations choose large frogs which should 
be well hardened in strong formalin (10 per cent.) before the 
dissection is attempted; it is well to remove the roof of the 
cranium in order to admit the formalin. These preparations are 
best kept in glass vials so that they may be viewed from all sides. 
Permanent preparations showing the spinal nerves and the 
sympathetic nervous system in situ are desirable. 
VI. HusToLocIcAL PREPARATIONS. 
The making of permanent histological preparations should 
hardly be attempted by a person without experience, except under 
competent guidance. For general methods of procedure in 
mounting whole objects and preparing serial sections, as well as 
for some special methods applicable to the frog, see Guyer, 
Animal Micrology (University of Chicago Press). The follow- 
ing hints may prove helpful. : 
Most of the material to be sectioned may be very. satisfactorily 
fixed in Zenker’s fluid, which should be used in abundance for 
from 6 to 24 hours depending on the size of the object. Wash 
in running water, or in frequent changes of water, for from 12 to 
24 hours; to insure thorough washing it is well to keep the object 
for sevetal days in weak formalin (3 per cent. to 5 per cent.), 
changing the formalin as often as it becomes discolored, until the 
object changes from a yellow color to drab. The washing should 
be done in the dark (cover the material with a black cloth). It 
is important to remember that in all cases of fixation in a solution 
containing corrosive sublimate, the material must be treated with 
70 per cent. alcohol containing iodin, in order to remove the 
crystals. 
For staining sections, two general methods are available: 
(a) The sections may be stained on the slide with a nuclear stain 
(e. g., Delafield’s heemotoxylin), and counterstained on the slide 
wtih a cytoplasmic stain (e. g., Orange G or eosin). Orange G 
is to be preferred to eosin since the latter may fade in time, but 
