436 PATTERNS AND PROBLEMS OF DEVELOPMENT 



simpler animals are inductors of gradients, and the gradients are the real 

 organizers. Since the concepts of inductors and organizers have developed 

 from experiment on embryonic stages, it is necessary to raise the question 

 whether, or to what extent, similar factors are concerned in the action of 

 embryonic inductors and the inductors concerned in reconstitutional de- 

 velopment of isolated parts of adult animals. Experimental data bearing 

 directly on this question from the embryonic side concern chiefly certain 

 echinoderms, fishes, amphibia, and birds; but embryonic development of 

 various other forms affords some data which are also suggestive or posi- 

 tive. 



INDUCTION IN ECHINODERMS 



Before turning to the experimental data attention is again called to a 

 few points in the earlier discussions of echinoderm developmental pattern 

 (chap. vii). The hypothesis of two opposed, overlapping gradients of 

 concentration of substances, an "animal" and a "vegetal" gradient, the 

 one decreasing basipetally, the other acropetally, advanced by Runn- 

 strom and accepted and developed by his co-workers, Horstadius and 

 Lindahl, has been discussed (pp. 241, 243). A third, ventrodorsal gradient 

 has also been postulated, and Horstadius has suggested two opposed gra- 

 dients in the right-left axis.' Since animal and vegetal gradients overlap 

 according to this hypothesis, they must be specifically different; Runn- 

 strdm (1933) and Lindahl (1936) have advanced further hypotheses con- 

 cerning the character of animal and vegetal metabolisms. Moreover, to 

 account for their experimental results these authors find it necessary to 

 assume that one of these gradients in the polar axis may "suppress" the 

 other more or less completely, that both may be altered in extent or other- 

 wise, and that certain external agents affect one or the other specifically. 



Von Ubisch has arrived at somewhat different conclusions." He also 

 postulates existence of two substances, ectodermal and entodermal, de- 



1 References p. 241, footnote 20. In discussion of the experiments of these authors it will 

 be convenient to follow their usage of the terms "animal" and "vegetative," except that 

 "vegetal" is used instead of "vegetative." However, these terms are antiquated and have 

 little or no meaning for echinoderm development, except as indicating position or direction in 

 relation to the polar axis. The terms "apical" and "basal," and "acropetal" and "basipetal," 

 seem equally applicable and somewhat more convenient and are used except where the others 

 seem to be required. The terms "pole" and "antipole" have been used by some authors for the 

 two ends of the polar axis, but"polar" and "antipolar" are less satisfactory than "apical" 

 and "basal," and the use of "polar" to indicate one pole or the region adjoining that pole, 

 instead of the entire physiological axis, is likely to lead to confusion. 



2 Von Ubisch, 1925a, b; 1929; 1931; 1932(1, b; 1933; 1934; 1936a, b; 1938a. 



