Allen, Association di the Guinea Pig. 343 



were placed in a temperature of 40° C, 24 hrs. Washed well in 

 numerous changes of tap water about 12 hrs. In differentiat- 

 ing no attempt was made to complete the permanganate decol- 

 orization at once, but the sections were allowed to remain in 

 the permanganate from 15-25 sees., washed in distilled water, 

 then placed in the oxalic-acid-sulphite mixture for several min- 

 utes, or until differentiation ceased to be apparent. Then they 

 were washed in distilled water, replaced in permanganate 

 a short time and the former process repeated. The best results 

 were obtained when the tissues were carried back and forth 

 three or four times. Subsequent washing in tap water was 

 very thorough. 



The sections of the spinal cord were made at the level of 

 (i) the sixth cervical nerve roots and ganglia; (2) the eighth 

 thoracic nerve roots ; and (3) the third lumbar nerve roots. 

 The levels were chosen thus in order that the sections might 

 pass through the largest parts of the cervical and lumbar intu- 

 mescentiae, and through the smallest region of the thoracic 

 cord. 



Sections of the encephalon were made (1) transversely, 

 perpendicular to the base of the brain in front, of the infundi- 

 bulum and behind the optic chiasm, being located accurately by 

 means of the tracts of the thalami ; and (2) in the case of the 

 cerebellum, sagittally, through the vermis. 



Three ages have been chosen for reproduction, and draw- 

 ings made with the help of a camera lucida. The magniiica- 

 tion of the half tones of the cord is 21 3/^ diameters ; of the en- 

 cephalon, 9^2 diameters. 



/. Description of Transverse Sections Through the Medulla 



Spinalis of the Guinea Pig at Birth. 



Cervical Level. 



A section through the cervical level at birch is reproduced 

 in fig. I, plate V. Reference to this figure will show that at birth 

 a large number of fibers are meduUated, and that the whole 

 area of the white substance is almost uniform in its coloration. 

 The gray substance is traversed by a large number of meduU- 

 ated fibers running in all directions. 



151] 



