1036 EMBRYOLOGY. 



surrounds the external depression, completely closes it, and constitutes the com- 

 mencement of the crystalline lens (Fig. 570, 2 /). The latter, thus formed, 

 presses on the primitive eye- vesicle, pushes it backwards (2), and gives rise to a 

 secondary eye-vesicle, the anterior wall of which becomes the retina, and the 

 posterior wall the choroid. The lens is, therefore, a dependency of the epithelial 

 lamina ; the cells composing it become elongated into fibres in the centre, and 

 are disposed at its circumference as a primary amorphous envelope— the crystalline 

 capsule — and afterwards as a secondary envelope rich in vessels. 



The portion of the integument which is not doubled to form the lens envelops 

 the globe, and gives rise to the sclerotica and cornea ; the epidermis 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 Uke 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 Remak, 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. A\iditory 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 /ossa— which is gradually 

 developed, and at last becomes a closed cavity. At this time, the wall of the 

 labyrinth is only a simple epithelial membrane ; this is soon covered, outwardly, 

 by a vascular connective membrane that separates into three layers — an internal, 

 joined to the epithelium to form the membranous labyrinth ; an external, that 

 lines the cartilaginous labyrinth ; and a middle, the soft, embryonic, connective 

 tissue of which disappears, and is replaced by the perilymph. At the same time that 

 these changes of structure are occurring, the vesicular form of the labyrinth is 

 modified, and shows the cochlea, semicircular canah, utriculus, and sacculus. 



The middle and external ear arise from the first pharyngeal (branchial) cleft, 

 which is never completely closed, while the others disappear. At first, then, we 

 find a cavity communicating externally with the pharynx ; this cavity contracts, 

 and is separated into two portions by a partition that spreads across its middle, 



