EMBRYONIC INDUCTORS AND ORGANIZERS 497 



ing factor may apparently have different effects on the same ectodermal 

 region at different developmental stages. 



The general gradient pattern of the body is apparently a factor in the 

 results of heterotopic transplantations. The farther ventral or posterior 

 to the otic region the place of origin of ectoderm transplanted to the 

 otic region, the less otic development takes place. Transplants of pre- 

 sumptive otic ectoderm develop more completely at a level between ear 

 and eye than at levels some distance posterior and ventral to the otic 

 region, and the most complete development at the anterior level occurs 

 at an earlier host stage than at the more posterior and ventral levels 

 (Yntema, 1933). The anterior level apparently attains a condition favor- 

 able to, or less inhibitory to, ear development earlier than the posterior 

 and ventral levels. These data certainly suggest definite relation to the 

 anteroposterior and dorsiventral gradient pattern. The differences in otic 

 development in ectoderm from other than the otic region have commonly 

 been regarded as indicating differences in potency, but they may be 

 merely differences in susceptibihty or reactivity to the inducing factor. 

 Otic development in regenerating tail tissue, following implantation of 

 optic primordium or presumptive medulla, suggests, first, that the re- 

 generating tissue has undergone a considerable dedifferentiation and pos- 

 sesses, or has regained, potencies supposedly lost, and, second, that even 

 here different embryonic regions may serve as inductors. 



It is generally agreed that the otic vesicle induces aggregation and 

 flattening of mesenchyme cells about itself and that these give rise to the 

 cartilaginous capsule. Thai this mesenchymal reaction is not primarily 

 a specific determination and may be altered by change in conditions is 

 emphasized by Filatow (1927). He regards it as not widely different 

 from the inflammatory reaction following implantation of a foreign body 

 or, in later stages, even of an otic vesicle. Under influence of an implanted 

 otic vesicle primordia of other organs — a vertebra (Yntema, 1933), and 

 in the fish, Acipenser, shoulder girdle (Filatow, 1930) — show more or 

 less capsular reaction. However, it appears that the capsular reaction 

 may be absent when otic primordia are transplanted to other than the 

 normal otic region.''^ This induction may be chemical in character, as far 

 as accumulation of mesenchyme is concerned; but, in view of the wide 

 occurrence of cartilage development, it can scarcely be regarded as specific 

 for the otic vesicle, except perhaps mechanically in determining form of 

 the capsule. 



Development of the amphibian tympanic membrane is normally de- 



48 L. S. Stone, 1926; Luther, 1927; Balinsky, 1929; Kaan, 1930; Yntema, 1933. 



