APPENDIXES 735 



developmental stages of sea urchin, KCN; Galigher, 1921(7, developmental stages of 

 the sand dollar Dendraster, KCN, NH4OH. Vertebrates: Hyman, 1921, teleost 

 embryos, KCN, NH4OH, 19266, cyclostome embryos, NH4OH, CH3COOH; Bellamy, 

 1919, Bellamy and Child, 1924, frog embryos, KCN, HgCL^, NH4OH, ethyl alco- 

 hol, low temperature; Cannon, 1923, frog embryos, KCN, HgCLj; Buchanan, 

 1926c, chick embryos, HCN; Hyman, 19270, h, chick embryos and hearts, KCN, 

 NH4OH, NaOH; Hinrichs, 1927, chick embryos, ultra-violet. Two papers are com- 

 parative studies of susceptibility of animals from different groups to a series of related 

 agents or to a single agent: J. W. MacArthur, 192 1, susceptibility to basic and some 

 acid dyes of Paramecium, Dileptus, Hydra, Diigesia, several rhabdocoels, and micro- 

 drilous oligochetes; Hinrichs, 19246, ciliate protozoa. Hydra, Dugesia, microdrilous 

 oligochetes, ultra-violet and visible light after sensitization by eosin. For general 

 discussions of differential susceptibility see Child, 1914/, 19206, 19236, 19246, pp. 

 76-80, i928(/; Hyman, 19266, pp. 112-15; Watanabe and Child, 1933; Buchanan, 

 1930, 1935. Unpublished data on Volvox show the same death gradient with KCN, 

 ethyl alcohol, HCl, hypertonic solutions, NaCl, sea water, ultra-violet, visible light 

 with sensitization by eosin, and lack of oxygen in darkness. Numerous other publica- 

 tions concerned with action of external agents on living organisms give incidental 

 evidence of differential susceptibility. This seems to be particularly the case with work 

 on irradiation by ultra-violet. X-rays, and radium. Often, however, the use of only 

 one or a few concentrations or intensities which are found to produce an effect, and of 

 temporary exposure, make it difficult to determine whether, or to what extent, the 

 results represent differential inhibition, differential tolerance, or differential recovery. 

 The work directly concerned with the problem of differential susceptibility began 

 with the discovery of differences in survival time of physiologically young and old 

 planarian individuals and of death gradients and tolerance gradients within the indi- 

 viduals, resulting from exposure to certain concentrations of ethyl alcohol and KCN 

 (Child, igiie; 1913a, 6, d; 19146). Further investigation indicated that these differ- 

 ences in susceptibility were related to real dift'erences in physiological condition; and it 

 began to appear probable that study of susceptibility to cyanide in different organisms, 

 particularly in unicellular forms — forms consisting of a single cell series, such as many 

 algae, eggs, embryos, and other small organisms — might afford data of interest and per- 

 haps serve to indicate regional physiological differences which could not otherwise be 

 determined in small organisms. In view of the fact that cyanides and KCN had been 

 shown to be powerful inhibitors of many physiological oxidations, a general relation 

 between the differences in susceptibility to cyanide and differences in rate of respira- 

 tion or oxidation seemed highly probable. For this reason KCN was used extensively 

 in the earHer experiments, with the result that an axial differential susceptibility, as 

 indicated by the progress of cytolysis, disintegration, and death along the axis was 

 found to occur in the forms examined, both plants and animals. In the attempt to 

 throw more light on the problem data concerning respiratory metabolism and the 

 effect of KCN upon it were obtained for different regions of the body and for indi- 

 viduals in different physiological condition, which had been found differently sus- 

 ceptible to KCN. The earliest respiratory data on planarians were comparative 

 estimations of CO2 production on pieces from different body-levels by means of the 

 Tashiro biometer (Child, 1913a; Behre, 191 8). The Tashiro apparatus has been super- VIlX 



