866 



DEVELOPMENT 



[CH. IJX. 



on the outside of the optic cup, and the iris by the growing forwards 

 of the anterior edge of the optic cup. The ciliary processes arise from 

 the hypertrophy of the edge of the optic cup, which forms folds into 

 which the choroidal mesoblast grows, and in which blood-vessels and 

 pigment-cells develop. 



The iris is formed rather late, as a circular septum projecting 

 inwards, from the fore part of the 

 choroid, between the lens and 

 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 

 capsulo-pupillaris (fig. 665). It 

 is supplied with blood by a 

 branch of the arteria centralis 

 retince, which, passing forwards 

 to the back of the lens, there 

 subdivides. The arteria centralis 

 is obliterated in the adult, and is 

 then called the canal of Stilling. 

 The membrana capsulo-pupillaris 

 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 Bar. Very early in the development of the embryo a 

 depression or ingrowth of the epiblast 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 laby- 

 rinth of the internal ear, consisting of the vestibule and its semicir- 

 cular 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 membranous labyrinth, the 

 bony semicircular canals, the walls of the cochlea with its scala vesti- 

 buli and scala tympani. 



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



FIG. 665. Blood-vessels of the capsulo-pupillary 

 membrane of a new-born kitten (magnified). The 

 drawing is taken from a preparation injected by 

 Tiersch, and shows in the central part the con- 

 vergence of the net- work of vessels in the pupil- 

 lary membrane. (Kolliker.)j 



