138 INDUCTION AND ORGANISATION 



cells that will really take part in the formation of the ear- 

 vesicle; the peripheral parts of the ear-field have lost this 

 potency. 



For a long time it remained doubtful what, in normal devel- 

 opment, induces the ear-vesicle. On the one hand, Dalcq was 

 able to show that in Discoglossus there is a centre of ear- 

 vesicle induction in the lateral parts of the archenteron roof. 

 On the other hand, Trampusch (1941) made it seem likely that 

 an important role is played in the induction of the auditory 

 organ by a cell group lying in the neural folds during the 

 neural plate stage. Later, this group, the neural crests migrates 

 in a ventral direction under the ectoderm at the sides of the 

 head. Extensive experiments by Yntema (1950) have now 

 elucidated this problem, at least for Amblystoma 'punctatum. 

 According to his results, the determination of the auditory 

 organ takes place in two phases. At the late gastrula or early 

 neurula stage, a first, weak determination is effected by the 

 archenteron roof. From the late neurula stage onwards, a second 

 induction takes place, this time by the brain or the neural 

 crest. This gives the determination of the ear-vesicle a more 

 final character. There are qualitative differences between these 

 two inductions, and in the ectoderm two qualitatively different 

 phases of competence correspond to them. In other words, each 

 induction attains its maximum at about the same time at which 

 the ectoderm's ability to react also reaches its correspond- 

 ing peak. 



Finally we must mention that xenoplastic induction of the 

 auditory organ between anurans and urodeles is possible 

 (Holtf refer, 1935). Such a xenoplastic labyrinth develops 

 normally for a long time. The speed of its development, and 

 its size and proportions are entirely analogous to those in the 

 species to which the ear-forming ectoderm belongs. It may even 

 function normally in the foreign larva. At a certain stage in 

 larval life, however, an incompatibility reaction suddenly sets 

 in, and the xenoplastic labyrinth rapidly disintegrates (Andres, 

 1949). 



The anterior end of the archenteron, which was the first to 

 invaginate during gastrulation, comes into close contact with 



