ORIENTATION OF THE EAR VESICLE 35 



A I, A II, B I, F I. No endolymphatic appendage or semicircular 

 canals are formed in any of these specimens. Accordingly, a decision 

 as to the posture of the vesicles is difficult to reach. 



E II. No vesicle. 



DISCUSSION 



In explanation of the environmental influences which cause the 

 inverted ear vesicles to rotate back to their normal position, 

 Streeter suggests three possibilities which may act either sepa- 

 rately or in combination in bringing about this phenomenon. 



First, the shape of the vesicle and the pocket into which it 

 fits: ''The auditory pocket is bounded by the different struc- 

 tures, which present different degrees of resisting pressure and 

 act upon the ear vesicle from various directions. \\'Tien the ear 

 vesicle is abnormally placed the pressure compels the vesicle to 

 turn back and keep the normal equilibrium. An objection to 

 this explanation is the fact that vesicles having abnormal form 

 and that could not possibly fit well in the usual pocket, right 

 themselves almost as well as the normally shaped ones. " This 

 objection is hardly justified, since the vesicle might have turned 

 back before it differentiated, as shown by findings in experiments 

 upon early stages. The inverted ear vesicles in the early stages 

 often are not of the regular oval form. Notwithstanding the fact 

 that this irregularity would serve to prevent the vesicle from 

 fitting into the original pocket, there were still many cases in 

 which such vesicles did regain their normal position. Thus, 

 according to my results, mechanical force alone cannot be regarded 

 as the cause of rotation. 



The second hypothesis advanced by Streeter is that the nerve 

 fibers which connect the ganglion mass with the brain may serve 

 to draw the vesicle into its proper position. My studies have 

 demonstrated that the ear vesicle can complete its rotation at a 

 stage when the nerves have not yet grown out from the ganglion, 

 which is closely attached to the vesicle ; accordingly, there is no 

 direct connection between the brain and the vesicle. This, 

 therefore, cannot be considered as the sole cause of rotation. 



The third possibility is mutual influence between the vesicle 

 and the environment, direct or indirect, in a chemotactic sense. 



