432 AN INTRODUCTION TO ZOOLOGY 



At a later stage the lens withdraws from the ectoderm, leaving a 

 space representing the anterior or aqueous chamber. The super- 

 ficial ectoderm becomes thinner and transparent, forming the 

 conjunctiva. Thus it will be seen that the parts of the eye essential 

 to vision, i.e. the conjunctiva, the lens, the retina and the optic 

 nerve, are derived directly from the ectoderm as in the case of the 

 first two, or indirectly from it, via the brain in the case of the second 

 two. All the remaining or accessory parts are derived from the 

 mesoderm. 



Ear. 



At thirty hours, or soon after, a thickened patch of ectoderm 

 appears on each side of the hind-brain, just in front of the first pair 

 of somites and nearly above the hyoid arch. These are the auditory 

 plates, the rudiments of the membranous labyrinths. These plates 

 increase in size and sink in the middle to form the auditory pits, 

 whose edges approximate until by the third day we find a large 

 auditory sac, formed and connected with the surface by a narrow 

 tubular canal. This canal is the endolymphatic duct, homologous 

 with the similarly named structure in the dogfish. At the end of the 

 third day it loses its connection with the exterior, and so the auditory 

 sac comes to lie freely in the head mesenchyme. Some days later 

 the distal end of the ductus enlarges to form an endolymphatic sac, 

 that finally extends along the mesoderm above the dorsal-lateral 

 surface of the myelencephalon. The auditory sac elongates dorso- 

 ventrally, and soon an internal ridge appears, marking its division 

 into a dorsal portion, the utriculus, and a ventral region, the sacculus. 

 At first the end of the duct opens into the upper corner of the' 

 utriculus, but this part subsequently expands dorsally on the outer 

 side of the ductus, which thus comes to open into the inner side of the 

 utriculus. 



About the fifth day three narrow grooves appear in the superior 

 chamber in the relative positions of the semicircular canals. They 

 grow out as thin hollow expansions, their inner margins fuse save 

 at two ends, thus converting them into tubular canals opening at 

 each end into the utriculus. Gradually they move outwards, carry- 

 ing along the line of fusion of their edges a thin sheet of the sac 

 wall with them, but with the breaking down of this sheet and the 

 enlargement of one end of each to form an ampulla they become 

 transformed into typical semicircular canals. The sacculus gives 

 rise to an outgrowth that later develops into the cochlea. At first 

 the walls of the auditory sac are moderately thick, but with its 

 enlargement they become much thinner save in certain areas, which 

 mark the positions of the maculae, cristee and papillae of the adult ear. 



