370 PATTERNS AND PROBLEMS OF DEVELOPMENT 



small tarsal claws but no muscles or articulations. These data suggest 

 an ectodermal gradient with high end at the tip, at least in earlier stages, 

 and mesodermal development progressing from the proximal region dis- 

 tally (Child and Young, 1903). 



In the regenerating amphibian leg skeleton and skin develop, even 

 though they have been more or less completely removed from the proxi- 

 mal stump." The longitudinal half of the leg of Triton does not regener- 

 ate the half removed; but the distal part, regenerated from a transverse 

 surface of section of such a half-leg, may be complete and normal as 

 regards polarity and asymmetry." The distal portion of a leg may re- 

 generate from the proximal cut surface of part of a leg implanted by its 

 distal cut end (p. 390). Apparently the primary polar pattern of the re- 

 generate is not determined by the part from which it develops but origi- 

 nates in the budlike outgrowth. However, the mitotic index in earlier 

 stages of regeneration is highest near the base and shifts distally as re- 

 generation progresses (Litwiller, 1939). Also, structural differentiation, 

 practically all mesodermal, progresses from the proximal region distally. 

 Except for these data, nothing is known concerning gradient pattern in 

 the regenerating or the embryonic amphibian leg. Undoubtedly there is 

 a radial gradient system, decreasing from a center in the early stages; 

 but whether the high central region becomes the distal tip or remains 

 proximal or the high region becomes distal in the ectoderm and proximal 

 in the mesoderm, or whether the gradient pattern changes in the course of 

 original development and regeneration, remains to be determined. Early 

 stages of appendages in the chick embryo show dye reduction decreasing 

 radially from a center, but information concerning dye reduction in earlier 

 stages of the amphibian leg is lacking, partly because of pigmentation 

 and the difhculty of staining with the oxidized dyes. 



In all these cases of reconstitutional development of a new axis in rela- 

 tion to section it originates either as a direct alteration of a pre-existing 

 axis by a new dominance and gradient, as in Tubularia and Corymorpha, 

 or as an outgrowth of new tissue much like a bud in that developmental 

 activity at first decreases more or less radially from a center and the 

 radial decrement becomes longitudinal by differential growth. In hydroids 

 the radial symmetry of the new axis may originate anew in the bud or 

 may develop in relation to the radial pattern of a stem piece. For ex- 



" Weiss, 1925, 1927a; Bischler, 1926; Liosner, Woronzowa, und Kusmina, 1936; and cita- 

 tions by these authors. 



"Weiss, 1925c, 19266. See also Graper, 1926a, b. 



