PYRAMID TRACT IN SQUIRREL AND CHIPMUNK 143 



and ]Vluller's fluid for the first two or three days and then trans- 

 fers it to pure Muller's fluid or 3 per cent potassium bichromate 

 solution, changing several times. He also prefers the stronger 

 solution for staining — osmic acid one part, Muller's fluid two 

 parts. In my experience formaUn fixation gives uncertain re- 

 sults. 



After removal from the Marchi fluid the pieces were washed 

 for several hours in running water, then placed in 75 per cent 

 alcohol, carried up the alcohol — xylene — paraftin series, and un- 

 bedded and cut in paraffin. In some cases celloidin was used as 

 the imbedding material. 



The entire brain from the mesencephalon to the lower end 

 of the medulla oblongata, including the first cervical segment of 

 the cord, was cut into serial sections, all of which were mounted. 

 This gave series which were not quite continuous, however, 

 since there was always some unavoidable loss in trinmiing the 

 surfaces of the pieces into which the brain had been originally 

 divided, prior to immersion in the Marchi fluid. Sections were 

 also mounted from each segment of the spinal cord. 



RE8ULTS OBTAIN 1:D 



Red squirrel 



Since the lesion was practically the same in all cases (fig. 1), 

 it will be sufficient to describe in detail the results obtained in 

 one individual of each species. As a matter of fact, it was found 

 that the areas of degeneration at corresponding levels agreed 

 closely in all the animals although some slight differences exist, 

 particularly in the region of the decussation. All the figures refer 

 to the same individual and were drawn from the preparations 

 with the help of a Gage projection apparatus. 



Transverse section of mesencephalon through anterior corpora 

 quadrigemina [fig. 2). At the level of the exit of the third nerve 

 the crusta shows degeneration over the entire area of its trans- 

 verse section, but the lateral and mesial extremities contain fewer 

 fibers than the middle portions. The gray matter of the sub- 

 stantia nigra contains many transversely cut fibers immediately 



