. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 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 Labyrintki; Origin of the Semicircular Canals. We have already seen that the shifting of the ductu
. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 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 Labyrintki; 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 w^all 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 clay; 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
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