8o PATTERNS AND PROBLEMS OF DEVELOPMENT 



tivation following section, which is apparently the primary factor in de- 

 termining the development which follows, or by inhibition of the resulting 

 growth or differentiation, or by both. The results as regards cathodal and 

 anodal inhibition in different species and with different current densities 

 may perhaps mean that, in the one case, one of these activities, in the 

 other case, the other, is inhibited. But aside from the question of inter- 

 pretation, an electric-potential gradient is certainly characteristic of the 

 polar axis of plants and animals, as far as investigated, parallels closely 

 other gradient expressions, and appears to be correlated with metabolic 

 activity; but many questions and problems await further investigation. 



In many animals galvanotactic reactions show a definite relation to the 

 polar gradient. The reaction, however, cannot be regarded as a simple at- 

 traction of unlike poles resulting from an electric gradient in the organism 

 but is a physiological reaction to an external differential and can be al- 

 tered or reversed by change in environment or physiological condition."^ 



OTHER METHODS CONCERNED WITH PATTERN 



Other biophysical, biochemical, and histochemical methods give evi- 

 dence of differential distribution or regional localization of substances in 

 relation to developmental pattern. As differentiation progresses, many 

 such localized differences of course appear as consequences of the earlier 

 pattern; but some quantitative differentials, in content of water, fat, car- 

 bohydrate, etc., may persist or appear in relation to axiate pattern in adult 

 individuals. The nitroprusside reaction indicates a polar glutathione gra- 

 dient in some forms; in others no differential has been found. A modifica- 

 tion of the nitroprusside reaction has been used by J. Brachet (1938) as an 

 indicator of differential distribution of SH-proteins in amphibian develop- 

 ment. 



Evidence concerning early developmental stages bears more directly 

 on the problem of pattern. A differential localization of sulphydryl pro- 

 teins in very definite relation to developmental pattern in the amphibian 

 egg and embryo has recently been reported (J. Brachet, 1938). By stain- 

 ing and injecting eggs with acid-alkali indicators the appearance of a polar 

 differential in hydrogen-ion concentration at the time of polar-body for- 

 mation has been observed in eggs of certain annelids, mollusks, and tele- 

 osts, resulting finally in some eggs in sharply defined acid and alkaline 

 zones.'^ The question of distribution and disappearance of glycogen has 



•^Hyman and Bellamy, 1922; A. R. Moore, 1923; Hyman, 1932/;. 

 '7 Spek, 1930, 1933, 19341', ^' ^- 



