SECT. 3] 



AND ORGANISATION 



591 



Since the death and disintegration of the different parts of the 

 body usually follow a regular time sequence, Child found it possible 

 to determine the time not merely of disintegration of the whole 

 animal, but of various regions of the body. The method used as a 

 rule was to examine the lot of animals at intervals of half-an-hour 

 and then to record the condition of each individual. Arbitrary values 

 having then been attached to the various stages of disintegration, 

 curves can be constructed showing the rate at which the process 

 has gone on. Fig. 97, taken from 

 Child's 1915 book, shows such a 

 curve constructed for specimens 

 of a flatworm, Planaria dorotoce- 

 phala, the curve ab representing 

 the susceptibility of young and 

 the curve cd representing that 

 of old animals. The correlation 

 between age and susceptibility 

 should be noted. 



The indirect method involves 

 the principle of acclimatisation. 

 In general, according to Child, 

 the ability of an animal to 

 acclimatise itself to cyanide or 

 other toxic agent varies with its 

 metabolic rate, or rather its level 

 of physiological gradient. The 

 ability of parts of animals to ^^' ^^' 



become acclimatised also alters with the same variable, so that the 

 indirect method affords a way of estimating gradients in the embryo. 

 The relation between age and survival time in solutions of the 

 concentration necessary for the indirect method is exactly the converse 

 of what it was in the direct method, for here the younger animals with 

 the higher metabolic rate live much longer than the older ones with the 

 lower metabolic rate. 



The susceptibility method is obviously a very complicated one, 

 and conclusions from experiments with it have to be drawn with 

 caution. In lethal doses which are not concentrated enough to pro- 

 duce death within a short time after the beginning of the exposure, 

 the regions of higher activity are always affected first, so that above 



Hourso 1 



