DEVELOPMENT OF THE SENSE ORGANS 



383 



duct corresponds to that of selachian fishes, which remains open to the exterior. 

 In man, its dorsal extremity is closed and dilated to form the endolymphatic sac 

 (Fig. 366 e). 



The differentiation of the auditory vesicle has been described by His, Jr. 

 and more recently by Streeter (Amer. Jour. Anat., vol. 6, 1906). In an em- 

 bryo of about 7 mm. the vesicle has elongated, its narrower ventral process con- 

 stituting the anlage of the cochlear duct (Fig. 366 a). The wider dorsal portion 

 of the otocyst is the vestibular anlage and it shows indications dorsally of the de- 

 veloping semicircular canals. These are formed in n mm. embryos as two 

 pouches, the anterior and posterior 

 canals from a single pouch at the 

 dorsal border of the otocyst, the 

 external canal later from a lateral 

 outpocketing (Fig. 366 d}. The mar- 

 gins of these pouches are thickened, 

 but elsewhere their walls are flattened 

 together and fused to form an epi- 

 thelial plate. Three such epithelial 

 plates are produced and internally 

 about the periphery of each plate 

 canals are left communicating with 

 the cavity of the vestibule. Soon the 

 epithelial plates are resorbed, leaving 

 spaces between the semicircular epi- 



Coch/ear an/aye 



FIG. 365. Transverse section through the 

 right half of the hind-brain and through the right 

 otic vesicle showing the position of the endolym- 

 phatic duct. From an embryo 6.9 mm. long (His). 



thelial canals and the vestibule (Fig. 



366 c). Dorsally a notch separates 



the anterior and posterior canals. Of these the anterior is completed first, next 



the posterior canal. The external canal is the last to develop. 



In a 20 mm. embryo (Fig. 366 e) the three canals are present and the coch- 

 lear duct has begun to coil like a snail shell. It will be seen that the anterior and 

 posterior canals have a common opening dorsally into the vestibule, while their 

 opposite ends and the cranial end of the external canal are dilated to form 

 ampulla. In each ampulla is located an end organ, the crista acustica, which will 

 be referred to later. By a constriction of its wall the vestibule is differentiated 

 into a dorsal portion, the utriculus, to which are attached the semicircular canals, 

 and a ventral portion, the sacculus, which is connected with the cochlear duct 

 (Fig. 366 e,f). At 30 mm. the adult condition is more nearly attained. The 



