EMBRYONIC INDUCTORS AND ORGANIZERS 459 



THE LATERAL DIFFERENTL\L IN INDUCING CAPACITY 



Inducing capacity of the dorsal lip region before invagination decreases 

 laterally from the median region, and the same is true of the inductor 

 after invagination; the somite region is less effective than the chorda 

 primordium, and the lateral mesoderm still less effective." The lateral de- 

 crease in ability to induce neural plate is steeper in anura than in urodeles; 

 lateral and latero ventral parts induce tail, not neural plate (G. A. 

 Schmidt, 1936a, b; Schechtman, 1938). There is, in short, a gradient in 

 inducing power, decreasing from the median region laterally; susceptibil- 

 ity of the dorsal lip region shows an essentially similar gradient (p. 152). 

 In this connection an experiment of Waddington's (1936a) is interesting. 

 He finds that transplanted, newly formed lateral-plate mesoderm is able 

 to induce an extra neural plate from presumptive ectoderm. Median dor- 

 sal lip region, substituted for presumptive lateral plate, also induces an 

 extra neural plate; but lateral mesoderm does not induce a neural plate 

 in normal development. These results suggest a relation of dominance and 

 subordination between higher and lower levels of the mediolateral gra- 

 dient of the presumptive chorda-mesoderm, the high median level being 

 dominant. When isolated from this dominance, the lateral region is able 

 to induce, perhaps because of some degree of activation following isola- 

 tion, as in invertebrate reconstitution. Lateral symmetry or asymmetry 

 of the inductor does not determine symmetry or asymmetry of the in- 

 duced plate. A right or left half of the dorsal lip region induces a. whole 

 neural plate, not a right or left half; and median or lateral pieces less 

 than half also induce a whole plate, though not always a completely sym- 

 metrical one. The implanted lateral half of the inductor from an early 

 gastrula reconstitutes to a bilaterally symmetrical system with median 

 chorda. Halves from more advanced stages show less reconstitution, the 

 notochord developing more or less at one side, and the other side being 

 formed by more or less appropriation (induction) of host tissue. The later 

 the stage of the implant, the more asymmetrical is the induced neural 

 plate.'' These facts again suggest that the mediolateral differences are pri- 

 marily quantitative gradient differences but become increasingly specific 

 with progress of development. 



" Bautzmann, 1926, 1928, 19296, 1933. See also Ruud, 1925; Mangold und Seidel, 1927; 

 and various other papers include evidence on this point. 

 '^ Spemann, 1918; Weber, 1928; B. Mayer, 1935. 



