IX 



EYE, OTOCYST 



343 



constricted so that a sac or vesicle is formed, lying beneath the ectoderm 

 and communicating with the exterior by a narrow tubular channel. 

 This sac is the otocyst. With further development the otocyst under- 

 goes a complicated process of modelling whereby it takes on the form 

 shown in Fig. 143. The most striking feature in this process is that the 

 wall of the dorsal portion of the otocyst comes to project as three 

 prominent plate-like structures arranged in planes perpendicular to one 

 another. These plates become obliterated except along their outer edges, 

 the marginal portion of each persisting as a tube curved in the form of 



a.v.c 



pv.C. 



View of the left otocyst of a Dogfish (Acanthias) as seen from the left side (after Retzius). 

 a, Ampulla ; a.v.c, anterior vertical canal ; e.dt endolymphatic duct ; h.c, horizontal canal ; 

 /, lagena ; p.vx, posterior vertical canal. 



an arch and opening into the main cavity of the otocyst at each end. 

 These are the semicircular canals and they are named according to their 

 position — external or horizontal (Fig. 143, h.c), anterior vertical (a.v.c) 

 and posterior vertical (jj.v.c). While the planes of the three canals are 

 perpendicular to one another, the plane of the anterior vertical of one 

 side of the body is parallel to that of the posterior vertical on the other 

 side. Each canal becomes dilated at one end to form a rounded swelling — 

 the ampulla (Fig. 143, a). These remarkable semicircular canals are an 

 arrangement for the more perfect carrying out of the primary function 

 of otocysts — the balancing of the body and the perceiving of changes of 

 position. The existence of three canals in planes at right angles is to 



