GRADIENTS, FIELDS, AND DETERMINATION 287 



patterns of fore-leg and hind-leg stumps, for this asymmetry pattern pre- 

 sents a differential or gradient pattern at the cut surface of the stump 

 which may be imposed on the regenerating tissue and perhaps also deter- 

 mines the asymmetry pattern of the transplant. But the regenerating 

 outgrowth on a limb stump is not limited or definitively determined in its 

 earlier stages as regards development into a limb, for it may develop into 

 a tail-like structure if transplanted to the tail region (Guyenot, 1927; 

 Guyenot et Ponse, 1930). According to Weiss (1927^), the regenerating 

 outgrowth of an amputated tail transplanted to a Hmb stump may de- 

 velop as hmb. In these cases, also, the symmetry pattern of the tail or the 

 asymmetry pattern of the limb is probably the factor determining the 

 character of development. However, Liosner and Woronzowa^ find a con- 

 siderable specificity in transplants of muscular tissue from tail to limb, etc. 



Implantation of the limb region with more or less rotation in the dorsi- 

 ventral plane from normal orientation is sometimes followed by rotation 

 to normal orientation.'^ This reorientation apparently occurs in relation 

 to the region from which the shoulder girdle develops, not to the body as 

 a whole, and has been regarded as in some way associated with the shoul- 

 der girdle. A small implant {ih somites) which develops no shoulder girdle 

 and a large implant (5 somites) which develops a complete girdle do not 

 rotate, but an implant of intermediate size (3I somites) which develops 

 part of a girdle may rotate. If a 3^-somite region and a ring of tissue 

 representing the region around it are separated and implanted with inde- 

 pendent and different rotation, the limb region rotates to normal orienta- 

 tion with the ring. The question arises whether the girdle as such or the 

 gradient pattern is the factor determining rotation. Undoubtedly, both 

 the asymmetry gradient pattern of the part undergoing rotation and that 

 of the part on which it rotates must have attained a certain degree of de- 

 velopment in order that rotation may occur. The fraction of the pattern 

 in the i^-somite piece may be insufficient to bring about reaction, and the 

 5-somite piece may fail to rotate because of physical conditions associated 

 with its size or because it possesses sufficient pattern to alter adjoining 

 regions to some extent instead of rotating in reaction to them. 



It appears highly probable, in view of the data, that the gradient sys- 

 tem or field about the transplanted or regenerating limb primordium does, 

 or may, influence it and impose pattern on it, or determine postural rota- 

 tion. The fact that an anteroposterior asymmetry of the limb coincides 



* Liosner and Woronzowa, 1937; Liosner, 1938; Woronzowa, 1938. 

 'Harrison, 1921a; Nicholas, 1924, 1925, 1926. 



