THE FCETUS. 



909 



Fig. 435. 



and constitutes the commencement of the crystalline lens (Fig. 435, 2, /). 

 The latter, thus formed, presses on the primitive eye-vesicle, pushes it 

 backwards (2), and gives rise to a secondary eye-vesicle, whose anterior wall 

 becomes the retina, and the posterior wall the choroid. The lens is, 

 therefore, a dependency of the 

 epithelial lamina ; the cells com- 

 posing it become elongated into 

 fibres in the centre, and are dis- 

 posed at its circumference as a 

 primary amorphous envelope 

 the crystalline capsule and after- 

 wards as a secondary envelope 

 rich in vessels. 



The portion of the integu- 

 ment which is not doubled to 

 form the lens envelops the 

 globe, and gives rise to the 

 sclerotica and cornea; the epi- 

 dermis furnishes the epithelium 

 of the latter, which becomes 

 distinct from the sclerotica in 

 the course of the fourth month. 



An aperture the sclerotic 

 cleft is made at the lower part 

 of the fibrous envelope of the 

 eye ; this is connected with the 

 development of the vitreous body : 

 a prolongation of the derm a 

 kind of conjunctival bud passing 

 through this opening, and placing 

 itself between the crystalline lens and the anterior wall of the secondary 

 eye-vesicle (3, gl), where it is developed and transformed into the vitreous 

 body. At first it is encircled by vessels like the lens, but these disappear 

 before the termination of gestation. 



The optic nerve is developed in the pedicle uniting the eye-vesicle to the 

 anterior cerebral vesicle. The retina is formed by the inner lamina of the 

 secondary eye-vesicle; it extends to the lens in changing its character 

 anteriorly. According to Eemak, the choroid is constituted by the posterior 

 lamina of the eye-vesicle ; it advances at first to the lens, and then becomes 

 inflected in front to form the iris. The borders of the pupil are attached 

 to the vascular envelope of the lens, and this gives rise to the pupillary 

 diaphragm, which disappears before birth. 



The motor and protective organs of the eye are gradually developed 

 around the globe. The eyelids are small cutaneous folds that appear 

 towards the first third of uterine life ; they increase, and unite at their 

 margin until immediately before, or soon after birth, when they separate. 



The lachrymal gland is a dependency of the epithelial lamina which is 

 pushed in above the globe. At first solid, it gradually becomes channeled 

 out by cavities, from which arise the excretory ducts. 



2. AUDITORY APPARATUS. The internal ear, auditory nerve, and middle 

 ear, are developed separately. The labyrinth appears in the shape of a 

 vesicle, which is not in direct relation with the posterior cerebral cell ; it is 

 constituted by a depression of the epidermic lamina, the auditory fossa, which 



LONGITUDINAL SECTION OF THE EYE OF EMBRYO 

 FOWL. 



1, From an embryo at about the 65th hour of incu- 

 bation ; 2, From an embryo a few hours older ; 

 3, From an embryo at the 4th day of incubation, 

 h, Corneal lamina ; /, Lens in Fig. 1, still con- 

 nected with the corneal lamina, and possessing 

 a small cavity, o, in its interior (in Figs. 2 and 

 3 it is seen detached, but still hollow) ; r, Intro- 

 verted portion of the primitive optic vesicle, sub- 

 sequently becoming the retina ; a, Posterior part 

 of the optic vesicle, which, according to Remak, 

 probably becomes the choroid coat, ciliary pro- 

 cesses, and iris, and in Figs. 1 and 2 is still con- 

 nected with the brain by the hollow optic nerve ; 

 a, Thickening of the corneal lamina around the 

 spot from which the lens has detached itself; 

 gl, Vitreous body, or humour. 



