CHAPTER II. 



KILLING. 



12. In the majority of cases, the first step in the prepara- 

 tion 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 discnssed 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 suflicient rapidity, both the death of the 

 organism and that of its histological elements. 



But tliis jnethod is by no means applicable to all cases. 

 There are many animals, especially such as are of a soft con- 

 sistence, 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 ])aralyse it by narcotics before killing- it. 



Sudden Killing. 



13. Heat. — The application of Heat affords a means of 

 killing suddenly. By it the tissues are more or less fixed at 

 the same time that somatic death is brought about. 



The difficulty consists in hitting off the right temperature, 

 which is of course different for different objects. I think 

 that 80° to 90° 0. will generally be amply sufficient, and that 

 very fi-equently it will not be necessary to go beyond 60° C. 

 An exposure to heat for a few seconds will generally suffice. 



Siiiiill nVijects (Protozoa, Hydroids. Brynzon) ra;iy lie liVDU^lit into a 



