498 PATTERNS AND PROBLEMS OF DEVELOPMENT 



pendent on the annular tympanic cartilage. When this is removed, the 

 membrane does not develop; when it is transplanted to other regions, 

 membranes form over it. This induction occurs in any region of the skin 

 before and after metamorphosis. The transplanted quadrate will induce 

 tympanic membrane, and the suprascapula shaped into annular form is 

 probably slightly inductive (Helff, 1938; 1934^^, b). 



Skin of fully developed dermal plicae with mucous and other glands 

 from postmetamorphic R. palustris transplanted to the backs of larvae 

 of R. cateshiana undergoes complete regression of the plicate structure to 

 generaHzed skin. Retransplanted after this regression to the otic region 

 of metamorphosing R. clamitans or R. cateshiana, it forms fully differen- 

 tiated tympanic membrane (Helff, 1934c). Whether, or to what extent, 

 mechanical or chemical factors, activating or specific, are concerned here 

 is not yet evident. 



According to the data, three successive inductions are involved in de- 

 velopment of the amphibian ear: the primary neural induction, induction 

 of the otic vesicle by hindbrain regions, and induction of the capsule by 

 the vesicle; probably other inductions are involved in later development 

 of its parts. But, as in case of the lens of at least some amphibians and 

 apparently of the neural plate, some degree of definition or determination 

 as the locus of particular developmental events occurs in the presumptive 

 otic region in relation to the general pattern of earlier stages before these 

 inductions take place. If that general pattern is a gradient pattern, the 

 otic region is primarily defined by its gradient co-ordinates, and induction 

 by hindbrain or other inductor may be merely an acceleration or intensi- 

 fication of changes already initiated by position in the gradient pattern. 

 The inductor, according to this view, is primarily an assisting factor in 

 bringing the reacting region to a physiological level at which a certain 

 kind of differentiation, already initiated, can continue to a certain result. 

 As inductor and reacting region differentiate, there may be definite spe- 

 cific relations between them. 



BALANCER, ORAL REGION, GILLS 



Early larval stages of most urodeles possess so-called ''balancers," slen- 

 der, elongated, tentacle-like organs, slightly club shaped at the tips, con- 

 sisting of epidermis and mesodermal core derived from the neural crest, 

 and situated ventral and somewhat posterior to the optic region. From 

 early neurula stages of Amhly stoma on, presumptive balancer ectoderm 

 can develop a balancer when transplanted alone to other head regions. 



