288 PATTERNS AND PROBLEMS OF DEVELOPMENT 



approximately in direction with the anteroposterior axis of the body and 

 is apparently the expression in the localized Hmb region of that pattern 

 is perhaps significant as indicating that polarity and asymmetry are not 

 fundamentally different in character; something constituting polarity in 

 the body as a whole determines an anteroposterior asymmetry in the 

 limb; similarly, something constituting dorsiventrality and bilaterahty in 

 the body determines a dorsiventral asymmetry in the limb. It seems diffi- 

 cult to account for facts such as these except in terms of gradient pattern. 

 Apparently, work is done in the postural rotation of an implant, and this 

 impHes a dynamic factor of some sort. 



Within the potency field of the limb development of a limb can be in- 

 duced not only by implantation of part of a limb bud but of an otic vesi- 

 cle, an olfactory placode, brain tissue, eye, or even a piece of celloidin 

 and also by a nerve deflected to a region of the field. The induced limb 

 appears later the farther posterior the position of the inductor and reac- 

 tivity to an inductor disappears from anterior to posterior regions of the 

 field progressively with advance of development.'" 



The inductor, whether implant or nerve, apparently serves in these 

 cases merely as a nonspecific activator, and its action seems to be pri- 

 marily on the mesenchyme. The developmental result is determined not 

 by the inductor but by the polar gradient or gradients resulting from the 

 locaHzed activation and outgrowth, the character of the field in which the 

 outgrowth takes place, and the asymmetry pattern representing a part 

 of the general body pattern. Either ectoderm or mesenchyme of the 

 axolotl limb bud at certain stages may determine limb development with 

 foreign mesenchyme or ectoderm, according to Filatow (1930a). This may 

 mean that both are activated sufficiently so that cither can act as a domi- 

 nant region. 



SOME OTHER FIELDS 



Experiment has shown the existence of various other fields in amphib- 

 ian development— ear, gills, urodele balancer, etc. — and in many of them 

 gradient characteristics appear. Like the limb and eye fields, the potency 

 field of the ear is more extensive than the differentiation field in earlier 

 stages. Otic development induced in other parts of the field than the 

 physiological center is more complete near the normal differentiation field 

 than at greater distances, and the anteroposterior axis is determined ear- 

 lier than the dorsiventral." Implantation of the presumptive ear region 



■" Detwiler, 1918; Balinsky, 1925, 1926, 1927, a, b, 1933, 19370; ^" Filatow, 1927. 

 " Yntema, 1933; Harrison, 1936a; Albaum and Nestler, 1937; Hall, 1937, 1939. 



