850 DEVELOPMENT. [CH. LVIII. 



the cornea. In the eye of the foetus of mammalia, the pupil is 

 closed by a delicate membrane, the membrana pupillaris, which 

 forms the front portion of a highly vascular membrane that, in the 

 foetus, surrounds the lens, and is named the membrana capsido- 

 pupillaris (fig. 669). It is supplied with blood by a branch of the 

 arteria centralis retinae, which, passing forwards to the back of 

 the lens, there subdivides. It is obliterated in the adult, and is 

 then called the canal of Stilling. The membrana capsulo-pupillaris 

 withers and disappears in the human subject a short time before 

 birth. 



The eyelids of the human subject and mammiferous animals, 

 like those of birds, are first developed in the form of a ring. 

 They then extend 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 separate. 



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

 depression or ingrowth of the e,piblast occurs on each side of the 

 head which deepens and soon becomes a closed follicle. This 

 primary otic 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 vesti- 

 bule 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 enclose this mem- 

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

 cochlea with its scala vestibuli and scala tympani. The auditory 

 nerve is gradually differentiated and grows towards the internal ear. 



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

 external auditory passage, are the remains of the first post-oral 

 cleft. The membrana tympani divides the cavity of this cleft 

 into an internal space, the tympanum, and the external meatus. 

 The mucous membrane of the pharynx, 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 structures are separated 

 only by the membrane of the tympanum. 



The pinna or external ear is developed from a process of 

 integument in the neighbourhood 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 de- 

 pression of the superficial epiblast at each side of the fronto-nasal 

 process (primary olfactory pit), which is at first completely 



