Devclopiinnit of A iii [yhibiaii Ear Vesicle 



435 



anterior and lateral canals are normal. In considering the posture 

 of the canals it is to be noted that the surrounding structures have 

 been left out in Fig. 2, to avoid unnecessary duplication; the three 

 models are all represented in the same relative position as that of 

 the labyrinth in Fig. i, /. e., the cephalic end is on the right, the 

 caudal end is on the left, the ventral surface is below, and the dor- 

 sal surface is above. Thus it will be seen that the lateral canals 

 in all three models are m the same plane; likewise the posterior 

 canals all form the dorso-caudal border of the labyrinth, and the 

 anterior canals form the dorso-cephalic border. The fact that 

 the anterior canal is small in model h,^ and the posterior canal is 

 small in model c, gives rise to a false impression of a backward 



sac. endolyinph. 



la gen a 



a ^ b c 



Fig. 2 Reconstructions showing the form and posture developed by three labyrinths one month old* 

 which while primitive ear vesicles were rotated from their normal position so as to lie face inward and 

 upside down. The models are placed so that their planes are parallel with those in Fig. i. Thus they 

 present a lateral view with the cephalic end toward the right, caudal end toward the left, dorsal surface 

 above, and ventral surface below. Enlarged 50 diameters. 



and forward tilting of the vesicle. The saccule and lagena have 

 the same position as in Fig. i, and the lagena points caudally as 

 it should do. The endolymphatic appendage lies on the median 

 side of the crus commune; in models b and c it is small, but the tip 



■• This may be due to injury received at the time of operation. Such localized defects are frequently 

 seen. They may involve any part of the labyrinth, and they vary greatly in the extent of the labyrinth 

 wall affected. In one case the entire labyrinth was defective, with the exception of the endolymphatic 

 appendage, which was normal in structure and position, and presented a curious appearance, being 

 attached to the small irregular vesicle representing the labyrinth. Such localization of abnormal 

 development is evidence of the high degree of specialization of the cells forming the primitive ear vesicle. 



