CHAP, x,] THE DEVELOPMENT OF THE CAT. 363 



invest it are formed from the mesoblast. The otic vesicle sends a 

 process inwards which becomes the scala media of the cochlea, which 

 again becomes separated off by a constriction (the future ductus 

 cochlearis) from the rest of the vesicle. From other parts of the otic 

 vesicle three rounded protuberances grow out, each of which becomes 

 first flattened and then absorbed, except at its circumference the 

 three protuberances thus becoming the three semicircular canals. 

 Two constrictions then show themselves in the wall of the large 

 part of the otic vesicle, thus separating the saccule from the part 

 adjacent to the semicircular canals which becomes the utricle. 



The auditory nerve is formed by direct transformation of the 

 mesoblast in contiguity with the otic vesicle. It is at first distinct 

 and separate both from that vesicle and from the hind-brain, though 

 it grows each way, and becomes connected with both. 



The cartilage which forms the auditory capsule (investing the 

 otic vesicle) is continuous with that of the basi-occipital region 

 the parachordal cartilage. This capsule becomes ossified from 

 three centres, the pro-otic, opisthotic, and epiotic bones, as before 

 described. These ossifications leave open two small apertures, one 

 in the outer wall of the cochlea, and surrounded by the opisthotic 

 bone thefenestra rotunda the other placed more inferiorly, oppo- 

 site the vestibule, it being bounded above by the pro- otic, and below 

 by the opisthotic thefenestra ovalis. 



The external meatus, tympanum, and Eustachian tube are formed, 

 as has been elsewhere said, by differentiation of the first visceral 

 cleft, while the auditory ossicles are formed as has been already 

 related. 



The sides and floor of the tympanum, and ultimately the floor of 

 the external auditory meatus, become ossified as the tympanic bones. 



From the margin of the second, or hyoidean, visceral arch, a 

 membrane grows out which becomes the external ear. 



The DEVELOPMENT OF THE NOSE resembles that of the eye, inas- 

 much as an involution of epiblast, the primitive nasal sac, is related 

 to an outgrowth of brain-substance, the olfactory lobe of the cere- 

 brum. Each of the nasal sacs deepens by outgrowth of its free 

 margin, but an inequality in the growth of that margin gradually 

 transforms it from a conical pit open forwards, into a groove or canal 

 leading backwards and inwards. Meantime the maxillary process 

 grows forwards and joins the naso-frontal process, thus bounding the 

 nasal sac below ; while the external lateral nasal process bounds it 

 behind intervening, as it does, between the nasal sac and the eye. 

 Thus the nasal sac comes to open behind into the front of the buccal 

 cavity, but ultimately (as has been already described) the develop- 

 ment (by lateral transverse growths) of the palate prolongs the nasal 

 canals backwards and so causes them to open into the hinder part of 

 the mouth. While this growth is going on, the ethmo-vomerine 

 cartilage (formed by the anteriorly coalesced trabeculre) grows down 

 and embraces the nasal sacs, sending down three cartilaginous pro- 

 cesses, one between and one on each side of them. The lateral 



