AXIAL SUSCEPTIBILITY GRADIENTS. 393 



visible by staining with neutral red before placing in KCN. 

 When death occurs the neutral red color changes to yellow, as 

 the alkali of the KCN solution penetrates the cells, and then 

 disappears. 



The death changes in ammonium hydrate are very similar to 

 those in KCN. The cells swell and in the blastula and gastrula 

 stages separate, but in later stages the cells cohere more or less 

 after death and the visible death changes are merely swelling 

 and rounding of the cells and a consequent increase in size and 

 greater translucency of the whole. Here likewise, return to 

 water increases the disintegration and so makes it easier to follow 

 the progress of death, and neutral red may also be used as an 

 indicator of death in the same way as with KCN. 



Ethyl alcohol is used in the same way as KCN and NH 4 OH, 

 but of course in much higher concentration. Disintegration of 

 blastula and gastrula stages is very complete and in the plutei 

 only the supporting tissues retain the body form. 



In hydrochloric acid, however, the behavior of the cells is 

 different, as might be expected. The cells shrink and do not 

 separate and death may occur with very little visible change 

 except in size. But return to sea water after a sufficient length 

 of time in HC1 brings about disintegration and the dead and 

 dying cells swell and separate so that the progress of death can 

 be followed without difficulty by removing lots from HC1 to 

 sea water at regular intervals. 



As regards concentration of the reagents used, a wide range is 

 possible according to the stage of development and the length 

 of survival time desired. Since the susceptibility increases very 

 greatly from fertilization up to the blastula stage as physio- 

 logical rejuvenescence occurs (Child, '15^, pp. 412-418), much 

 higher concentrations can be used for the former than for the 

 latter stages. At a temperature of 22-24 C. unfertilized eggs 

 begin to die after 4-5 hours in KCN m/ioo, while blastulse and 

 later stages begin to die after the same length of time in KCN 

 w/i,ooo. The other reagents were used only on the blastula 

 and later stages. In NH 4 OH m/^oo death of these stages begins 

 in half an hour to an hour, in m/i,ooo in 1-2 hours. The same 

 stages in alcohol 4 per cent, (roughly m 2/3) begin to die in 3-4 



