424 CHAPTER XXXIV. 



found after forty-eight hours. But it must be kept in the 

 dark. 



The following notes as to the proper duration of the hardening pro- 

 cess in different cases are taken from the papers quoted and other sources, 

 most of which may be found in v. LENHOSSEK, op. cit., p. 23. 



Spinal cord of chick, from the sixth to the tenth day of incubation- 

 twelve to forty-eight hours in the mixture (up to the fifth day the em- 

 bryos may be treated whole, later the vertebral column should be dis- 

 sected out and cut into two or three segments ; it need not be opened). 

 The spinal column of new-born rats and mice should be treated in the 

 same way, and remain in the mixture for twenty-four hours (for spinal 

 ganglia), or for two to six days for the cord itself. (The eiicephaloii of 

 these subjects may be treated in just the same way, without being dis- 

 sected out.) 



VON LENHOSSEK (op. cit., p. 10) recommends for human (foetal) cord 

 two to three days for neuroglia, three to five for nerve-cells, and five to 

 seven for nerve-fibres and collaterals. 



Cerebellum of new-bom subjects, three to five days in the mixture. 



Cerebral cortex of young subjects, two to three days (Mice), or as 

 much as five (Babbit, Cat) ; cortex of adults, eight to fifteen days. The 

 most favourable region of the brain is the cornu Ammonis, especially in 

 the Rabbit. 



Retina twenty-four to forty-eight hours in the mixture, then 

 " double " impregnation ( 815). 



Sympathetic three to seven days in the mixture, and two 

 in the silver : then double impregnation. 



Spinal cord of larvae of Amphibia. The entire Iarva3 

 (best 2 to 2'5 centimetres long) should be put for two to five 

 days into the mixture, and for one to two into the silver. 



Epidermis of Lwfribricusiloroe to six days in the mixture, and two 

 in the silver, or double impregnation if necessary. SMIRNOW makes 

 the mixture of equal parts of 5 per cent, bichromate and 1 per cent, 

 osmic acid, and leaves in it for five to twenty-eight days, and one to two 

 days in the silver (0'75 per cent.). 



Nervous system of Helix (glia-cells). The above mixture 

 for eight to ten days, then silver of 0*75 to 1 per cent. 



As a general rule, the younger the subject, the shorter should the 

 hardening be. If it has been too short, sections will have a brownish- 

 red opaque aspect, with precipitates, and irregular impregnation of cells 

 and fibres. If it has been too long, the ground will be yellow, without 

 precipitates, but with no impregnated element?, or hardly any. 



