286 PATTERNS AND PROBLEMS OF DEVELOPMENT 



progressively increasing regional specificity can account for the establish- 

 ment and development of this field. In Amblystoma the pattern of the 

 limb bud is determined in the anteroposterior direction of the body earlier 

 than the pattern in the dorsivejitral direction; and both of these before 

 the mediolateral, that is, the longitudinal or polar axis of the limb/' 

 Anteroposterior and dorsiventral axes are apparently determined about 

 the same time in anura and in Triton, but experiment on these forms is 

 less extensive than on Amblystoma.'^ Determination evidently follows 

 much the same course in the regenerating as in the original limb bud. 

 Apparently, the anteroposterior and the dorsiventral gradient pattern of 

 the body are imposed on the hmb primordium. In Amblystoma the dorsi- 

 ventral pattern is less effective and becomes fixed in the Hmb primordium 

 later than the anteroposterior. The longitudinal axis of the limb, how- 

 ever, is apparently determined, as in other buds that become axiate, by 

 the gradient system in the bud itself. The radial decrease in developmen- 

 tal activity about the physiological center becomes a longitudinal gradi- 

 ent as outgrowth occurs. 



That there is a real effective dominance in the limb field is suggested 

 by the fact that a limb region removed from its original position and im- 

 planted on the flank of the same or another animal is usually inhibited 

 or resorbed if within a certain distance of the limb regenerating from the 

 original site or of a normal developing limb, but develops if at a greater 

 distance (Detwiler, 1918; Hellmich, 1930). It might perhaps be expected 

 that the transplanted limb region, since it represents the region of highest 

 potency in the field, would dominate and inhibit regeneration from the 

 original site. Its failure to do this may result from depression brought 

 about by removal and implantation in a new environment, or the activa- 

 tion in the original site following its removal may be sufficient to inhibit 

 it. Even though polarity of the hmb in regeneration is not determined by 

 the proximal stump or by the potency field of the limb, the limb field 

 plays a part in determining the character of distal regenerating or trans- 

 planted parts. The regenerating outgrowth on the stump of an amputated 

 fore leg transplanted in an early stage to the stump of an amputated hind 

 leg may develop as hind leg, or that from a hind leg as fore leg when 

 transplanted to a fore-leg stump (Milojevic, 1924). It may be suggested 

 that the determining factor in this case is the difference in the asymmetry 



'' Harrison, 1921(7, b, 1925a; Svvett, 1926, 1927, 1928a, b, c, 1932, 1937a, b, 1938a, b, c, 1939; 

 Swett, 1936, experiments on hind limb. See also pp. 390-95. 



7 Graper, 1922a, b, 1923, 1924, 1925, 1926a, b; Brandt, 1924a, b, 1925; Milojevic, 1924. 



