Development of A nipliihian Ear Vesicle 433 



]:)KTHRMINATION OK POSITION OF THE EAR VESICLE 



The conclusion that the attitude of the developed labyrinth, the 

 position of its canals and various chambers, is determined by its 

 environment is based on seventeen experiments in which the ear 

 vesicle was loosened from its normal situation and placed in an 

 abnormal attitude, and the specimen then allowed to continue in 

 its development. At the end of a month examination showed that 

 the labyrinth had become differentiated with varying degrees of 

 completeness, and in each mstance had developed in normal rela- 

 tion to the surrounding structures. 



Rotation tn Two Directions. In eight of these experiments the 

 ear vesicle was rotated 180° around both its vertical and transverse 

 axes, so that it was turned face inward and upside down; or, in other 

 words, its lateral or invaginated surface was toward the brain and 

 its ventral border was where the dorsal border should be, the maxi- 

 mum displacement. After this procedure the wounds healed 

 within a few hours, and the larv.e were reared up to the fourth or 

 fifth week, when they were killed and cut in serial sections. The 

 labyrinths of hve specimens were reconstructed. Before describ- 

 ing them reference should be made to the normal condition of the 

 labyrinth at this age. A reconstruction of a normal one with its 

 adjacent structures is shown in Fig. i. 



From the reconstruction of a normal specimen it can be seen that 

 the three semicircular canals have individual characteristics by 

 which they can be separately identified; such as the Y-shaped 

 union of the anterior and lateral canals, and the overlapping of 

 the caudal end of the lateral canal by the posterior canal, and the 

 junction of the posterior and anterior canals to form the crus com- 

 mune. The diflPerentiation between utricle and saccule is not yet 

 complete, but the part that is to become saccule is so labeled. 

 From the caudal border of the saccule can be seen a small pocket 

 budding out which constitutes the lagena or primitive cochlea. 

 Directly median to the crus commune is the endolymphatic append- 

 age, consisting of a small duct leading from the main labyrinth 

 chamber up between the labyrinth and brain to a rounded pouch, 

 the saccus endolvmphaticus. In their histology, as well as in 



