PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 141 



in the apicobasal and ventrodorsal axes, basal and ventral regions being 

 most susceptible. According to his conclusions, water content and per- 

 meability to salts are highest in basal and ventral regions, and absence of 

 potassium brings about loss of water, first from apical and dorsal regions 

 of lower water content to the basal region and then from the whole egg 

 or embryo; decrease in dispersion of colloids is also believed to occur. The 

 differential susceptibility is regarded as resulting from differential per- 

 meability to potassium ions, decreasing from basal and ventral regions 

 acropetally and dorsally (see Appendix V, p. 742). According to Runn- 

 strom, the basal region is also more susceptible than the apical to high 

 concentrations of sodium thiocyanate, nile blue sulphate, and lithium 

 salts; development in a CO-O2 atmosphere in light shows injury in both 

 apical and basal regions, and evidence of a ventrodorsal and left-right 

 gradient is also found.^^ 



Most of Runnstrom's data on susceptibility are concerned with modi- 

 fications of development rather than with cytolytic or death gradients. 

 Differential modifications of development are considered in the following 

 chapter, but his views require some further consideration here in connec- 

 tion with the more direct evidence concerning gradient pattern. He ap- 

 parently regards the effects of external agents as more or less regionally 

 specific, some acting on the animal, others on the vegetal gradient, and 

 so on. It does not appear, however, that he has considered the possibility 

 of differential tolerance to external agents and of differential recovery 

 after temporary exposure. As will appear more clearly in the next chap- 

 ter, these reactions of the organism may bring about differential modifi- 

 cations of form and proportion opposite in direction, as regards axial rela- 

 tions, to those resulting from direct action of the agent. Nor does the 

 possibility that the gradient pattern or parts of it may undergo change 

 during development seem to have been considered. There is often little 

 evidence in his data of use of a wide range of concentrations and exposure 

 periods. Whether, or to what extent, some of the modifications described 

 represent differential inhibitions, differential tolerance, differential recov- 

 ery, or regionally specific effects seems, at present, uncertain. 



Differential cytolysis and death and differential dye reduction give no 

 evidence of overlapping, specifically different gradients in the polar axis. 

 They indicate a single polar gradient, the "animal" gradient in early 

 stages, and, according to the recent studies of Child (1936a), a second 

 "vegetal" gradient appearing later and partially obliterating or replacing 



■'^ Runnstrom, 1928a, b; 1929^; 1933; 1935a- 



