IOO THE FROG 



mal salt solution is one whose osmotic pressure with respect 

 to the tissue cells is equal to that of the plasma of the blood. 

 For use with the tissues of the frog this solution is made by 

 adding 6 grams of sodium chlorid to a liter of distilled 

 water. The slightly stronger solution used for the tissues 

 of mammals is not adapted for work with the amphibia. 



The making of permanent hisiological preparations should 

 hardly be attempted by a person without experience, except 

 under competent guidance. For general methods of pro- 

 cedure in mounting whole objects and preparing serial sec- 

 tions, see Guyer, Animal Micrology (University of Chicago 

 Press). The following hints may prove helpful. 



Most of the material to be sectioned may be very satis- 

 factorily 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 wash- 

 ing it is well to keep the object for several days in weak 

 formalin (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 black cloth). It is important to 

 remember that in all cases of fixation in a solution contain- 

 ing 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 hsematoxylin), and counterstained 

 on the slide with a cytoplasmic stain (e. g., a saturated aque- 

 ous solution of orange G, or a 0.5 per cent, aqueous solu- 



