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 Labyrinthi; 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). Shghtly 
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. Fie. 169.— Model of 
The horizontal pocket forms at about the mid- the auditory laby- 
dle of the external surface on the fifth day; mth (otocyst) of 
immediately above it is a roughly triangular, eniek emery e oF 
. ; , ee undetermined age ; 
pear-shaped depression in the wall of the oto- — yjew from behind. 
cyst, bounded by the vertical pocket on the (After Réthig and 
other two sides. Thus the vertical pocket con- — Brugsch.) 
sists of two divisions, anterior and posterior, C. 1., Pocket for 
: : Y ; the formation of the 
meeting at the apex of the otocyst (Fig. 169). lateral (horizontal) 
The pockets gradually deepen; and the eens Re 
semicircular canals arise from them by the fu- mation of vertical 
sion of the walls of the central part of each Ss Rae 
pocket, thus occluding the lumen except at of ductus cochlearis 
the periphery (Fig. 170). The fused areas 224 lagena. D. e., 
: 7 endolymphatic duct. 
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 
