CHAPTER II 

 KILLING 



10. In the majority of cases, the first step in the preparation 

 of an organ or organism consists in exposing it as rapidly and 

 as completely 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 can be extremely injurious to cells : in 

 the case of larger mammals like the cat and the dog a preliininary 

 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, because carbon monoxide does not appear 

 to be hurtful to cells. Amphibians 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 contract : or you may paralyse it by narcotics before 

 killing it. See also under " Chromosomes," § 621, and " Mito- 

 chondria," § 679. 



8. 



