I04 STRONG. [Vol. X. 



Preliminary to hardening, the manner of cutting the tadpole 

 is a matter of some importance. The special region should be 

 opened up to some extent in order to insure a speedy and com- 

 plete penetration of the osmium-bichromate and the silver 

 nitrate solutions, as neither of them have much penetrating 

 power. On the other hand, by not cutting into too small 

 pieces the precipitate formed by the silver is not deposited so 

 extensively on all the surfaces, and many beautiful pictures can 

 thus be obtained not marred by its presence. In investigating 

 the head region I have found it best to cut the animal trans- 

 versely into three pieces, one cut being made a short distance 

 caudad of the auditory capsule and Vagus, and the other passing 

 just through the anterior surfaces of the eyeballs. It is, per- 

 haps, best to allow the animal to remain in the fixing fluid half an 

 hour or so first, to acquire a better consistency for this cutting. 



With respect to the osmhwt-bichromate mixture, a consider- 

 able latitude in the proportions may be taken and good impreg- 

 nations obtained ; the same holds good as to the strength of 

 the silver nitrate solution. For example, good preparations 

 were obtained from a tadpole impregnated as follows : Pot. 

 bich. sat. sol. 140 cc. + osmic i^ 10 cc, 21 days, silver nitrate 

 10^, 3 days. The following, however, are the two solutions 

 chiefly used, the first one (i) being after Berkley (10) and the 

 second (2) very much like that recommended by Ramon y Cajal : 

 (i) Pot. bich. 5/0 84 cc. + osmic 2/0 16 cc, and (2) Pot. bich. 

 3j4fo 4 vols. + osmic ifo i vol. I cannot say that I have found 

 much choice between the two. 



As seen above, impregnations may be obtained after harden- 

 ing for 21 days in a solution containing /ess osmic. On the 

 other hand good impregnations of the more superficial parts 

 were obtained in one case as follows : Pot. bich. 3^^ 4 vols. 

 + osmic ifo I vol., 22 hours, during the first 11 hours in a 

 solution which had been previously used, silver nitrate ^/o, 

 2}4 days. This shows that here again there may be a con- 

 siderable range within which good results are obtainable. 

 There is a certain hardening period, however, which is much 

 the most favorable ; this is 2 to 5 days, and probably about 3 

 days will usually give the best results throughout the pieces. 



