496 PATTERNS AND PROBLEMS OF DEVELOPMENT 



rise to retina when brought into contact with epithehum of the developing 

 otic vesicle (Dragomirov, 1937; Ikeda, 1937). The thinning and loss of 

 pigment of the epidermis in development of the conjunctiva is induced 

 by an optic vesicle or cup, by a fragment of the retina, or by a lens. De- 

 velopment of conjunctiva from epidermis of other regions can be induced, 

 either by transplantation of optic vesicles or by grafting other epidermis 

 over the optic primordium, provided there is contact between the two.^s 

 Moreover, the reaction is not species-specific, at least not for various uro- 

 dele species (Mangold, 1929c; Harrison, 1929a), and a hmb bud implanted 

 in place of the optic cup at certain stages apparently induces conjunctival 

 development (Durken, 19 16), though this has been questioned (H. Peter- 

 sen, 1924). 



THE EAR 



Pieces of hindbrain or medulla implanted near the ear region or even 

 in the flank induce development of otic structures; this induction is effec- 

 tive between R. pipiens and yl. punctatum in either direction.-"^ Presump- 

 tive medulla, but not forebrain or spinal cord, implanted in regenerating 

 tissue of the tadpole tail, induces vesicles resembhng otic vesicles (H. S. 

 Emerson, 1939). The ability to induce ear is itself induced in presumptive 

 hindbrain by the underlying mesoderm, and this mesoderm can induce 

 ear development directly in foreign ectoderm in absence of hindbrain 

 (Harrison, 1935). The optic primordium induces otic vesicles among other 

 organs in regenerating tadpole tail tissue, but perhaps only indirectly 

 (Schotte, 1937). After removal of the presumptive otic region the ear 

 may develop from regenerated ectoderm or from ectoderm transplanted 

 from other body regions and even from anurans to Triton^'' On the other 

 hand, the earlier stages of ear development from presumptive otic ecto- 

 derm of neurulae are apparently independent of other organs, but induc- 

 tion is necessary for full differentiation (Kaan, 1926). If this is true, the 

 ear is primarily determined as a locus in the general pattern of the ecto- 

 derm without induction; but induction can determine ear development in 

 ectoderm of other regions, even in different species, genera, and orders of 

 amphibia, raising once more the question of specificity of the inducing 

 action. If otic induction is primarily an activation, it is activation at a 

 certain period of development when the reacting tissue has doubtless 

 attained a condition dift'erent from that of early stages. The same induc- 



« Fischel, 1900Z), 1917, 1919; Spemann, 1901a; W. H. Lewis, 1905; Groll, 1924. 

 ■i^L. S. Stone, 1931; Albaum and Nestler, 1937; Ponomarewa, 1936. 

 ^T Amblystoma: Tokura, 1925; Kaan, 1926; Yntema, 1933; Harrison, 1935, 1936, anura to 

 Triton: G. A. Schmidt, 1936c. 



