APPENDIX VI 



In his experiments on differential susceptibility with Raiia temporaria as material, 

 Cannon (1923) used only HgCU, and the only concentration mentioned in his paper is 

 m/i,ooo. According to his brief statement of results, disintegration showed no definite 

 relation to axes or other particular features of development. Entirely aside from the 

 question of differential susceptibility to different agents, it is highly improbable that 

 regional differences in susceptibility to a particular toxic agent in a developing embryo 

 can have no definite relation. In the light of the great body of definite positive evi- 

 dence of such relation these negative results must at least raise the question whether 

 the procedure employed is adequate, but it does not appear that this possibility was 

 considered by Cannon. In a re-examination of the matter Bellamy and Child (1924) 

 found it necessary to separate individual eggs from the mass, to remove the jelly, 

 and to agitate the solution. If these precautions were not taken, differences in suscepti- 

 bility might depend on differential exposure rather than on differences in physiological 

 condition. In the use of HgCl^ it must also be borne in mind that even rather low 

 concentrations act as fixing or coagulating agents and kill without any cytolysis. 

 Except for a surface effect on more advanced stages, apparently an action on the cilia 

 or the ciliated cells, Bellamy and Child found differential susceptibility to HgCL- es- 

 sentially similar to that observed with other agents. 



Cannon also calls attention to the fact, previously noted by Bellamy, that alcohol 

 produces changes in the basal yolk-laden region earlier than elsewhere. It was pointed 

 out by Bellamy and Child that this is evidently an effect of a lipoid-soluble and lipoid- 

 solvent agent on the yolk and has no relation to the differential susceptibility of the 

 protoplasm. In the amphibian egg the yolk content of the basal region is so great that 

 susceptibility of that region is more or less specific for fat-soluble agents. 



The argument advanced by Cannon that the disintegrating tissue is toxic and that 

 disintegration, once initiated at any point, will continue because of this toxicity is 

 shown by many facts to be without foundation. First, disintegration may be ar- 

 rested by return of the organism to the natural environment; and the parts which have 

 not disintegrated may recover and develop further, or, as in planarians, undergo re- 

 constitution. Second, when a locahzed area or gradient arises in some region other than 

 the end of an axis, for example, in an organ primordium, it may remain localized 

 or may progress in one direction and not in others, according to the pattern of the 

 organ concerned. Third, in cases of partial differential tolerance with death and dis- 

 integration of certain regions cessation of disintegration and finally reconstitution of 

 the parts which disintegrated may occur with continued exposure to the agent which 

 originally produced the disintegration (see pp. 113-17). 



In experiments on differential modification of cleavage by KCN Cannon found that 

 in Biifo toxicity decreased, and in Ra)ia increased, with decrease in concentration. 

 The frog eggs were removed from the ovary and fertilized by addition of sperm, while 

 the toad eggs were laid and fertilized naturally. The possibiHty that the higher con- 



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