172 GEORGE L. STREETER 



The development of a normally placed labyrinth from an in- 

 verted vesicle can only occur in one of two ways. Either the ear 

 vesicle at the time of operation consists of indifferent cells which 

 are capable of forming various parts of the labyrinth in accord- 

 ance with how they chance to lie ('harmonisch-aquipoten- 

 tielles System') ; or the ear vesicle itself rotates as a whole after 

 the operation, so that the cells originally intended for the differ- 

 ent parts are brought to lie in their correct relation, where they 

 continue in the fulfillment of their destined development. As 

 was clearly argued by Spemann '10, it cannot be explained by 

 the former. All our evidence points to a high degree of differ- 

 entiation of the cells of the vesicle, and it is conspicuously proven 

 by their possession of laterality, which has been described in the 

 earlier part of this paper. We could not otherwise have a left- 

 sided labyrinth on the right side of the head. 



This leaves us with the alternative that the displaced ear vesi- 

 cle does not stay in the position in which it is placed, but rotates 

 into a normal posture. Regarding the nature of the force that 

 produces the rotation there is yet little information. One must 

 take into consideration at least three possibilities which either 

 separately or in combination may explain its action. In the 

 first place, it may be an active phenomenon on the part of the 

 ear vesicle, that is, intrinsic motility of the vesicle itself; secondly, 

 it may be based upon some attraction existing between some por- 

 tion of the vesicle and the brain or other structure; and thirdly, 

 there may be some purely mechanical basis for it. 



In the first place, regarding the intrinsic motility of the ear 

 vesicle itself, we are familiar with the flowing motion of protoplasm 

 in the case of amoebal pseudopods, and Harrison ('10) has de- 

 scribed the remarkable movements of the protoplasmic processes 

 of nerve cells. In these instances there is a movement of one 

 part of a single cell in relation to the rest of the cell. In the case 

 of the ear vesicle, however, we should have to consider a mass- 

 movement of a group of cells. Such movements have already 

 been described for small masses of cells, such, for example, as the 

 lateral line rudiment, which, as has been shown by Harrison ('03) 

 migrates in the course of a few days, all the way from the head 



