500 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Transplanted gill ectoderm of Amhlystoma shows less evidence of gill de- 

 velopment (Harrison, 192 1&), but after removal of gill ectoderm and meso- 

 derm and implantation of a limb bud in the gill region some gill develop- 

 ment may occur in ectoderm adjoining and usually ventral to the gill 

 region (Detwiler, 1922). After removal of presumptive branchial ento- 

 derm in tail-bud stages the gill does not develop, and this entoderm, 

 transplanted beneath ectoderm and mesoderm slightly posterior and ven- 

 tral to the gill region, can determine gill development (Severinghaus, 

 1930; Ichikawa, 1934, 1938). Induction of gill development may occur 

 in presumptive neural plate or in ventral ectoderm transplanted hetero- 

 plastically to the gill region between Triton species (Spemann und Rot- 

 mann, 1931), or from axolotl to Triton (Rotmann, 1935c), or even xeno- 

 plastically, from an anuran, Bombinator igneus, to Triton (G. A. Schmidt, 

 1937&). The induced gills show donor characteristics more or less clearly 

 in time of appearance, size, time of degeneration, and, when differences 

 between donor and host are considerable, by lack of correspondence be- 

 tween gills and underlying mesentoderm. However, some modification by 

 the host of size and perhaps of period of persistence apparently takes 

 place. In general, the experimental results indicate that presumptive 

 branchial ectoderm of the neurula and tail-bud stages possesses some ca- 

 pacity for independent gill development, apparently less in urodele than 

 in anuran species; but for complete gill development the underlying ento- 

 derm or mesentoderm seems to be necessary. 



"double assurance" in amphibian development 

 Implanted pieces of the dorsal lip can induce development of a neural 

 plate from ectoderm entirely outside the presumptive neural region, and 

 induction by the underlying chorda-mesoderm is supposedly a factor in 

 neural-plate formation in normal development. However, as we have seen, 

 there is considerable evidence that the presumptive neural plate, or at 

 least its anterior part, is in some degree "determined," predisposed to the 

 course of development resulting in neural plate, and under certain condi- 

 tions can undergo more or less development along this line independently 

 of an underlying inductor. Similarly, lens development can be induced in 

 ectoderm of various regions by an optic cup; but in some amphibians a 

 lens can develop in the presumptive lens region quite independently of 

 an optic cup, and in others some degree of determination or predisposition 

 toward lens formation is apparently present in the presumptive lens epi- 

 dermis. These and other cases of apparently twofold determination Spe- 

 mann regards as examples of the principle of double assurance or the 



