VER TEBRA TES. 37 



hifflier forms ; and in the second ))lacc, the cochlea gradually acquires independence 

 fioia the sacculus, and eventually becomes converted (in the mammals) into a spirally 

 coiled tube, whence in fact it derives its name. 



It has been mentioned above that vibrations may reach the labyrinth in some sharks 

 by means of the first gill-cleft. In the smooth hound and similar forms there is a dis- 

 tinct recess of the cleft that stretches up towards the labyrinth. This is exactly what 

 we find in higher vertebrates; only the gill-cleft itself disappears, and the recess 

 stretches up from the cavity of the mouth in the form of the Eustachian tube, to ter- 

 minate under the skin in a dilatation — the tympanic cavity — which lies to the 

 outside of the osseous labyrinth. The portion of skin immediately over the cavity, 

 being thin and tense, is subject to vibration, and may be either quite ex])osed on the 

 side of the head, as in the frog, or at the bottom of a longer or shorter passage, the 

 external auditory meatus, as in higher foruis. An additional provision for collecting 

 the vibrations towards the tympanic membrane is afforded by the external eai', which, 

 however, reaches a high grade of development only in mammals. 



The question now arises how the chain of bones which serves to transmit vibra- 

 tions from the tympanic membrane to the labyrinth gets into the tympanic chamber. 

 We saw above that it represents in all probability the suspensorial apparatus of the 

 lower jaw in fishes, and further that the loose bones of the head in the lattei- group 

 form an important channel by which vibrations reach the labyrinth. We may con- 

 clude from this that the function which the chain of bones performs in the higher 

 vertebrates is inherited from lower forms, where they had, however, additional duty 

 to perform. If it be also agreed that the Eustachian tube and tym])anic cavity 

 represent the recess of the first gill-cleft in a shark, then we may conclude that the 

 higher vertebrates have inherited both the wave-transmitting recess and the chain of 

 bones from some fish-like ancestor. The ])osilion of the chain of bones in the tym- 

 panic cavity can only be explained by assuming that the dilated end of the recess 

 grows round the chain, envelojiing it in such a way as to bring them within the cavity, 

 but yet covering them on every side with mucous membrane as it does so. 



The Eye. 

 The essential difference in the develojnnent of the eye from that of the other 

 sense-organs — its neuro-epithelium being a direct outgrowth of the medullary epi- 

 blast — has been noted above. At a very early stage the first cerebral vesicle buds 

 out on either side the optic vesicle, which, however, is soon converted into an optic 

 cup by an ingrowth from the overlying skin to form the lens (Fig. 39). The lens is 

 purely epidermal in its origin, and the little piece of mesoblast which it ]nishcs before 

 it becomes transformed into the vitreous humor, a clear gelatinous tissue which tills 

 the cavity of the optic cup. The wall of the cup next to the lens bej-omes converted 

 into the retina, r (Fig. 41), the other wall remains thin, and in fact only formed of a 

 single layer of epithelium, but this acquires a distinctive chcractcr and an important 

 function, by a deposition of black pigment in its cells, wiiich then constitute the pig- 

 mentary ei)itlielium of the retina. The cavity separating the two layers of the 

 retina, at one time continuous with the thalamoccEle, disappears, as does that of the 

 optic stalk, which becomes converted into the optic nerve. Closely applied to the 

 retinal epithelium is the choroid coat, raesoblastic in origin, and serving chiefly as a 

 carrier of vessels for the nourishment of the eye, and of pigment. This coat has cer- 

 tain muscular fibres in it anteriorly (the ciliary portion), which jjartly serve to support 



