CHAPTER II. 



KILLING. 



12. IN the majority of cases, the first step in the preparation of. 

 an organ or organism consists in exposing it as rapidly and as com- 

 pletely as possible to the action of one of the Fixing Agents that are 

 discussed in the next chapter. The organ or organism is thus taken 

 in the normal living state ; the fixing agent serves to bring about at 

 the same time, and with sufficient rapidity, both the death of the 

 organism and that of its histological elements. 



It should be noted that narcotisation generally implies some 

 change in the cells, and most narcotics have to be applied for a long 

 time. Such treatment is absolutely barred in material destined for 

 careful cytological study. This applies especially to ether and 

 chloroform, which are extremely injurious to cells : in the case of 

 larger mammals like the cat and dog a preliminary treatment in 

 ether or chloroform may be necessary, but directly after anaesthesia 

 the animals' throats should be cut or they should be killed by a blow, 

 if possible. Coal gas chambers are good for killing all mammals, 

 and I do not believe carbon monoxide is hurtful to cells. Amphi- 

 bians killed by chloroform are often completely spoilt for cytological 

 purposes ; if the brain is not wanted, pith the animal. For birds 

 the time-honoured custom of wringing their necks is recommended. 

 In the case of small lizards, newts and such live stock it is a good 

 plan to cut off their heads quickly with strong scissors. If the 

 material is wanted for chromosome or mitochondria work look up 

 these sections for special directions. 



But these methods are by no means applicable to all cases. There 

 are many animals, especially such as are of a soft consistence, and 

 deprived of any rigid skeleton, but possessing a considerable faculty 

 of contractility, which if thus treated contract violently, and die in 

 a state of contraction that renders them unfit for study. In these 

 cases special methods of killing must be resorted to. Speaking 

 generally, there are two ways of dealing with these difficult cases. 

 You may kill the animal so suddenly that it has not time to con- 

 tract : or you may paralyse it by narcotics before killing it. See 

 also under " Chromosomes," 652, and " Mitochondria," 673. 



