342 Journal of Comparative Neurology and Psychology. 



the complexity in the psychical life of the white rat is wholly 

 out of proportion to the very few tangential fibers to be found 

 in the cortex. 



Technique. 



The guinea pigs used for histological study were of the 

 common English variety. From a large number of nervous 

 systems hardened and stained the following ages were selected 

 for examination : 



Each guinea pig chosen for study was in good physical 

 condition, the wide range of weight indicated in the table being 

 within the bounds of normal individual variation. Most of 

 those animals used in the psychological experiments were after- 

 ward killed for examination. 



The central nervous system was exposed and hardened vi 

 situ in Mullek's fluid. The tissue was kept in the dark during 

 the hardening process, which required about fifty days for the 

 small animals, and from sixty to seventy days for the adults. 

 The sections were embedded in celloidin, ten grades being 

 used,^ and were cut 21 // thick. They were stained according 

 to the Pal-Weigert haematoxylin method, modified slightly to 

 obtain the best results from this particular tissue. In detail the 

 modified method is as follows: After cutting sections in 70% 

 alcohol they were run to distilled water, then mordanted in 

 Muller's fluid 24 hours, at a temperature of 36 to 40° C. 

 Washed thoroughly in distilled water, 2 to 4 hrs. Fresh VVei- 

 gert's haematoxylin (cold) was poured over them, and they 



* Hardesty: Neurological Technique, p. 



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