5o6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Experimentation along these lines has given results of much interest 

 in relation to the problem of pattern, though it has not yet provided a 

 basis for complete agreement as to the character of that pattern. Discus- 

 sion of the regional differences in reconstitution in relation to the axiate 

 pattern of the sea-urchin egg is based largely on the extensive experiments 

 of Horstadius from 1928 on. Here again, use of the terms "animal" and 

 "vegetal" will be convenient in certain connections. Also, it will be re- 

 called that, according to the data on differential susceptibility and differ- 

 ential dye reduction, the apical or "animal" pole is the high end, the basal 

 or "vegetal" pole the low end, of the primary polar gradient. According 

 to the Runnstrom hypothesis, animal and vegetal poles are, respectively, 

 the regions of highest concentrations of animal and vegetal substances.'' 



Animal halves, the eight mesomeres of the sixteen-cell stage, or 

 on, + an, of a later stage (Fig. 145 [p. 438]) represent about two-thirds 

 of the presumptive ectodermal region. They usually give rise, when iso- 

 lated from basal halves, to apical partial forms without entoderm or mesen- 

 chyme but with variation in scale of organization from the extreme apical 

 tjqDe, with the long cilia normally characteristic of the apical region ex- 

 tending over most of the surface, to less extreme forms (Fig. 146, A-E 

 [p. 439]). Isolated awi rings, that is, approximately the most apical fourth, 

 become extreme apical partial forms, usually completely covered in earher 

 stages with the long ciha and later uniformly ciliated (Fig. 147, an, 

 [p. 441]). Reconstitution of isolated an, rings is less extremely apical and 

 very similar to that of the apical half (Fig. 147, an, [p. 441])- These iso- 

 lated apical halves and an, and an, are more apical in reconstitutional de- 

 velopment than in normal development. They may develop as extreme 

 apical regions, and some of them are apparently almost or entirely apolar 

 or anaxiate. According to Horstadius (1935), this is because the animal 

 gradient gains the upper hand over the vegetal; but how this takes place 

 is not made clear. Presumably concentrations of hypothetical animal and 

 vegetal substances are not immediately altered by isolation. What deter- 

 mines the assumed alteration? The results suggest that some effect of 

 regions farther basal is present in normal development of the whole em- 

 bryo but absent in the isolated apical parts. The observed primary polar 

 gradient, decreasing basipetally, and the activation in the basal region 

 preceding gastrulation, giving rise to a secondary gradient opposite in 

 direction to the primary and partly obliterating it (pp. 134-40) provide, 

 a somewhat different basis for interpretation of the experimental results. 



■» See pp. 241, 243, and chap, vi in general. 



