66 PATTERNS AND PROBLEMS OF DEVELOPMENT 



death gradient in the dye and in other agents: his results are confirmed 

 by many other observations on staining and toxic effects of various dyes.^ 

 In his discussion of the Uterature of vital staining MacArthur calls at- 

 tention to the apparent relation between rate of staining and physiological 

 condition of different regions and concludes from the evidence of his own 

 and other work that the axial staining gradient cannot be due to a differ- 

 ential permeability in any merely physical sense. The cell surface is living 

 and concerned in the activity of the cell. The staining and toxic effect of 

 the dye depend on its adsorption, precipitation, flocculation, or chemical 

 combination on or with cell constituents. Its continued entrance and ac- 

 cumulation within the cell depend on its removal from the solution in the 



Evidence from other work also indicates that graded differences in vital 

 staining, particularly in the simpler animals and earlier developmental 

 stages, are associated not merely with differences in permeability but with 

 graded differences in physiological condition of the cells. Numerous ob- 

 servations on vital staining of protozoa, blastulae, planulae, young hy- 

 droids, stages of reconstitution of Corymorpha and other hydroids, turbel- 

 laria, and microdrilous oligochetes agree with MacArthur's data in show- 

 ing that with sufficient staining axial staining gradients usually appear 

 which parallel closely the death gradients observed with other physical 

 and chemical agents. The gradients of injury, cytolysis, and death, with 

 further continued staining, coincide with the staining gradient. 



A point of some interest in connection with this differential staining is 

 that, when staining has progressed to a certain stage, the gradient of sus- 

 ceptibility to cyanide disappears, all levels being equally susceptible. 

 With further staining the susceptibility gradient undergoes reversal, that 

 is, the regions which were originally most susceptible to cyanide become 

 least susceptible after injury by the dye. This reversal of differential sus- 

 ceptibility to cyanide along an axis by deep staining has been observed 



" Protozoa, Child and Deviney, 1926; Hydra, Child and Hyman, 1919; embryonic, larval, 

 and adult hydroids. Child, i()i()d, 1926a; larval stages of the polyclad, Stylochus, Watanabe 

 and Child, 1933, and other turbellaria. 



* Hypotheses concerning the manner in which vital staining occurs are not lacking. Since 

 MacArthur's work is under discussion, it may be noted that he suggests an electrochemical 

 hypothesis. The cation of the dye, which is the color ion, unites with an intracellular anion, 

 which is a product of catabolism; if such anions are formed by dissociation of amphoteric pro- 

 teins as acids, their production should be decreased by acidity and increased by alkalinity and 

 by abundant o.xygen, as seems to be true. According to McCutcheon andLucke (1924), how- 

 ever, e.xternal alkalinity favors, internal alkalinity decreases, staining with basic dyes. They 

 suggest that these dyes combine with some acid cell constituent. 



