ORGANS OF SPECIAL SENSE 



291 



It is interesting to note that the epidermic attachment to the endo- 

 lymphatic duct is about at the junction of the saccus endolymphaticus 

 and ductus endolymphaticus s.s. If this may bear a phylogenetic inter- 

 pretation, it would seem that the saccus should be regarded as an addi- 

 tion to the primitive ductus of Selachii, which opens on the surface. 



(b) Development of the Pars Superior Lahyrinthi; Origin of the 

 Semicircular Canals. We have already seen that the shifting 

 of the ductus endolymphaticus to the median surface of the 

 otocyst is brought about by a vertical extension of the superior 

 lateral wall of the otocyst, which forms a shallow pocket opening 

 widely into the otocyst (Fig. 167). Slightly 

 later a second pocket is formed by a horizon- 

 tally extended evagination of the lateral wall 

 of the pars superior directed towards the 

 epidermis. These two pockets, known as the 

 vertical and horizontal pockets, are the fore- 

 runners of the semicircular canals : the vertical 

 of both anterior and posterior, and the hori- 

 zontal of the horizontal semicircular canal. 

 The horizontal pocket forms at about the mid- 

 dle of the external surface on the fifth day; 

 immediately above it is a roughly triangular, 

 pear-shaped depression in the wall of the oto- 

 cyst, bounded by the vertical pocket on the 

 other two sides. Thus the vertical pocket con- 

 sists of two divisions, anterior and posterior, 

 meeting at the apex of the otocyst (Fig. 169), 



The pockets gradually deepen; and the 

 semicircular canals arise from them by the fu- 

 sion of the walls of the central part of each 

 pocket, thus occluding the lumen except at 

 the periphery (Fig. 170). The fused areas 

 subsequently break through. The peripheries 

 thus form semicircular tubes communicating at each end with 

 the remainder of the superior portion of the otocyst, or utriculus, 

 as it may now be called. Three semicircular canals are thus 

 formed, one from each division of the original vertical pocket 

 and one from the horizontal pocket. The upper ends of the an- 

 terior and posterior semicircular canals, formed from the anterior 

 and posterior divisions of the vertical pocket, open together into 



Fig. 169. — Model of 

 the auditory laby- 

 rinth (otocyst) of 

 a chick embryo of 

 undetermined age ; 

 view from behind. 

 (After Rothig and 

 Brugsch.) 



C. 1., Pocket for 

 the formation of the 

 lateral (horizontal) 

 semicircular canal. 



C. v., pocket for for- 

 mation of vertical 

 semicircular canals. 



D. C, Primordium 

 of ductus cochlearis 

 and lagena. D. e., 

 endolymphatic duct. 



