THE ORGANS OF SPECIAL SENSE. 593 



At this stage (embryos of 4 to 5 mm.) the auditory vesicle is an oval or 

 spherical sac the wall of which consists of two or three layers of undifferen- 

 tiated epithelial cells. It lies against the neural tube and is connected with the 

 latter by the acoustic ganglion (Fig. 512, a). About the same time an evagina- 

 tion appears on the dorsal side of the auditory vesicle, forming the anlage of the 

 endolymphatic appendage (Fig. 512, a, b, c). The evagination continues to 

 elongate and comes to form a club-shaped structure, the distal end of which 

 becomes flattened to form the endolymphatic sac, the narrower proximal portion 

 constituting the endolymphatic duct (Fig. 512 a-n). The epithelium, which at 

 first consisted of two or three layers of cells, becomes reduced to a single layer. 

 In the chick the endolymphatic appendage is formed out of the original union 

 between the ectoderm and the auditory vesicle (Keibel, Krause). In Reptiles 

 and Amphibia (Peter, Krause) and in man (Streeter), on the other hand, this 

 appendage develops independently of the union, appearing on the dorsal side of 

 the seam of closure in the auditory vesicle. 



In embryos of about 6 mm. the auditory vesicle (apart from the endolymph- 

 atic appendage) becomes differentiated into two portions or pouches a bulging, 

 triangular one above, which is connected with the endolymphatic appendage, 

 and a more flattened one below. The former is the vestibular pouch, the latter 

 the cochlear pouch (Fig. 512, b-f). Between the two is a portion of the vesicle 

 which is destined to give rise to the saccule and utricle, and which may be called 

 the atrium (Streeter). Properly speaking, the atrium is a division of the 

 vestibular pouch. The cochlear pouch is phylogenetically a secondary diver- 

 ticulum which develops from the atrium, appearing first in the lowest land- 

 inhabiting Vertebrates (Amphibia). 



As mentioned above, the vestibular pouch early assumes the form of a 

 triangle, with the apex toward the endolymphatic appendage. The three 

 borders of the triangle form the anlagen of the semicircular canals and bear the 

 same interrelation as the latter. At the same time a vertical groove (the lateral 

 groove) appears between the anlage of the posterior canal and the posterior end 

 of the lateral canal (Fig. 512, b, d). 



The formation of the semicircular canals is shown in Fig. 512, g-k. The 

 edges of the triangular vestibular pouch expand and become more or less 

 crescentic in shape. The two walls in the concavity of each crescent come 

 together and then break away (Fig. 512, g, j, absorp. focus), thus leaving the rim 

 of the crescent as a canal attached at its two ends to the utricle. The breaking 

 away affects first the superior, then the posterior, and finally the lateral canal. 

 During these gross changes the epithelium becomes reduced to a single layer 

 of cells. 



At one end of each canal an enlargement appears to form the ampulla, as 

 shown in Fig. 512, /, m, n, and Fig. 513, a, b, c. 



