APP.] HARDENING EMBRYOS. 425 



of the jet as required, a very steady mean temperature 

 may be maintained. 



In the absence of gas, a patent night-light placed at a 

 proper distance below the bath may be made to answer 

 very well. When a body of water, once raised to the 

 necessary temperature, is thoroughly surrounded with 

 non-conducting material, a very slight constant amount of 

 heat will supply all the loss. 



B. On preparing sections of the embryo. 

 1. HARDENING. 

 a. Picric acid. 



We find this reagent the most satisfactory 

 for hardening the chick and in most instances 

 mammalian embryos. 



Klein enberg's solution of picric acid is the 

 best. 



With 100 parts of water, make a cold 

 saturated solution of picric acid ; add to this 

 two parts of concentrated sulphuric acid or 

 nitric acid : filter and add to the filtrate three 

 times its bulk of water. 



In this solution of picric acid 1 the embryo 

 must be placed and left for from 2 5 hours. 

 It should then be washed in alcohol of 30 p.c. 

 and placed in alcohol 50 p.c. for one hour. 

 From this it must be removed into alcohol 

 of 70 p.c. in which it should be left until 

 all the picric acid is extracted ; to facilitate 

 this the 70 p.c. alcohol should be frequently 

 changed : when free from picric the embryo 



1 It is sometimes advantageous to add to this solution of picric 

 acid as much pure kreasote as it will dissolve (vide Kleinenberg, 

 'Development of Earthworm," Quarterly Journal of Mic. Sci. 1879). 



