II. THE PERIOD OF ORGAN DEVELOPMENT 137 



These phenomena were further investigated by Harrison 

 (1936). Initially, the potency to form ear-vesicle is not strictly 

 limited to the material that will later actually produce it. It 

 extends also over the surrounding ectoderm, though there is a 

 gradual decrease in intensity. Any cell group from this zone 

 can produce a complete ear-vesicle after transplantation, e.g. 

 into the ventral side. The size of such an ear-vesicle depends 

 upon that of the graft, but its structure may be entirely normal. 

 Several ear-vesicles can originate from one "prospective ear 

 region" in this way, and, conversely, two such regions can fuse 

 if grafted side by side. In the latter case, they produce one 

 ear-vesicle of double size. This proves that here, too, we find an 

 organisation-field possessing a certain freedom with respect to 

 the cell material. 



Harrison has also investigated how the symmetry of the 

 auditory organ is determined. Each of these organs is in itself 

 asymmetrical, for it has different shapes in the three dimen- 

 sions. But one auditory organ is symmetrical to that of the 

 other side of the body. It is an important problem how the 

 determination into a right or left-side organ takes place. As 

 we shall discuss this more fully in connection with the limbs, 

 the following remarks will suffice here. Initially, the ear 

 primordium is still unpolarized, so that after transplantation it 

 develops into a left or a right ear in accordance with its new 

 environment. But soon the rostrocaudal axis, and then also the 

 dorsoventral axis, become fixed. Once this has happened, these 

 axes cannot be reversed any more, and when inverted the 

 primordium will go on developing in accordance with its original 

 polarity. From then on, a left primordium will always become 

 a left auditory organ, even if transplanted into the right side. 



At a still later stage the determination of the component 

 parts within the "ear-field" takes place. After this, a grafted 

 part of the field no longer produces a complete auditory organ, 

 but only certain parts of one. Exchange of cell groups now 

 results in ear-vesicles of abnormal composition (Kaan, 1926). 

 This shows that the ear-field is no longer a unity, but that it is 

 broken up into a mosaic of parts with different potencies. 

 Moreover, the ear-forming potency is now restricted to those 



