V.] THE EAR. Ill 



In front of the line of the ora serrata, both walls of the 

 optic cup, quite thin and wholly fused together, give rise to 

 the pigment-epithelium of the ciliary processes and iris, the 

 bodies of both these organs being formed from the meso- 

 blastic investment. 



12. During the second day the ear first made its appear- 

 ance, on either side of the hind-brain as an involution of the 

 external epi blast, thrust down into the mass of mesoblast 

 rapidly developing between the epiblast of the skin and that of 

 the neural canal (Fig. -15, au.p.). It then had the form of a 

 shallow pit with a widely open mouth. Before the end of the 

 third day, its mouth closes up and all signs of the opening are 

 obliterated. The pit thus becomes converted into a closed 

 vesicle lined with epiblast and surrounded by mesoblast. 

 This vesicle is the otic vesicle, whose cavity rapidly enlarges 

 while its walls become thickened (Fig. 32, (7(7). 



The changes by which this simple otic vesicle is converted 

 into the complicated system of parts known as the internal 

 ear, have been much more completely worked out for 

 mammals than for birds. We shall therefore reserve a full 

 account of them for a later portion of this work. Meanwhile a 

 brief statement of the main course of events in the chick may 

 be useful ; and will be most conveniently introduced here, 

 although we shall have, in doing so, to speak of changes 

 which do not occur till much later than the third day. 



The internal ear consists essentially of an inner mem- 

 branous labyrinth lying loosely in and only partially attached 

 to an outer osseous labyrinth. 



The membranous labyrinth (Fig. 33) consists of two 

 parts: (1) the vestibule, with which are connected three pairs 

 of semicircular canals, pag', fr, hor, and a long narrow 

 hollow process, the aqueductus or recessus vestibuli, and 

 (2) the ductus cochlearis, which in birds is a flask-shaped 

 cavity slightly bent on itself. The several parts of each of 

 these cavities freely communicate, and the two are joined 

 together by a narrow canal, the canalis reuniens, cr. 



The osseous labyrinth has a corresponding form, and may 

 be similarly divided into parts : into a bony vestibule, with 

 its bony semicircular canals and recessus vestibuli, and into 

 a bony cochlea ; but the junction between the cochlea and the 

 bony vestibule is much wider than the membranous canalis 

 reuniens. 



