DEVELOPMENT. 813 



those of birds, are first developed in the form of a ring. They then ex- 

 tend over the globe of the eye until they meet and become firmly 

 agglutinated to each other. But before birth, or in the carnivora after 

 birth, they again separate. 



The Ear. Very early in the development of the embryo a depres- 

 sion or ingrowth of the epiblast occurs on each side of the head which 

 deepens and soon becomes a closed follicle. This primary optic vesicle, 

 which closely corresponds in its formation to the lens follicle in the eye, 

 sinks down to some distance from the free surface; from it are developed 

 the epithelial lining of the membranous labyrinth of the internal ear, 

 consisting of the vestibule and its semicircular canals and the scala media 

 of the cochlea. The surrounding mesoblast gives rise to the various 

 fibrous bony and cartilaginous parts which complete and inclose this 

 membranous labyrinth, the bony semicircular canals, the walls of the 

 cochlea with its scala vestibuli and scala tympani. In the mesoblast 

 between the primary optic vesicle and the brain, the auditory nerve is 

 gradually differentiated and forms its central and peripheral attachments 

 to the brain and internal ear respectively. According to some authori- 

 ties, however, it is said to take its origin from and grow out of the hind 

 brain. 



The Eustachian tube, the cavity of the tympanum, and the external 

 auditory passage, are remains of the first branchial cleft. The mem- 

 brana tympani divides the cavity of this cleft into an internal space, 

 the tympanum, and the external meatus. The mucous membrane of 

 the mouth, which is prolonged in the form of a diverticulum through 

 the Eustachian tube into the tympanum, and the external cutaneous 

 system come into relation with each other at this point; the two mem- 

 branes being separated only by the proper membrane of the tympanum. 



The pinna or external ear is developed from a process of integument 

 in the neighborhood of the first and second visceral arches, and probably 

 corresponds to the gill-cover (operculum) in fishes. 



The Nose. The nose originates like the eye and ear in a depression 

 of the superficial epiblast at each side of the fronto-nasal process (pri- 

 mary olfactory groove), which is at first completely separated from the 

 cavity of the mouth, and gradually extends backward and downward till 

 it opens into the mouth. 



The outer angles of the fronto-nasal process, uniting with the max- 

 illary process on each side, convert what was at first a groove into a 

 closed canal. 



THE ALIMENTARY CANAL. 



The alimentary canal in the earliest stages of its development con- 

 sists of three distinct parts the fore and hind gut ending blindly at 



