738 



THE EYE. 



projection from below upwards between the lens and the optic vesicle, in such a 

 manner as to invaginate the superficial and lower walls of the vesicle, pressing them 

 upwards and inwards on the superior and deep walls, and giving them the form of a 

 cup imperfect below, the secondary optic vesicle. The involution gives rise to the 



(Fig. 491. DIAGRAMMATIC SKETCH OP A 

 VERTICAL LONGITUDINAL SECTION 

 THROUGH THE EYEBALL OP A HUMAN 

 FCETUS OP FOUR WEEKS (after Kolli- 

 ker). 122 



The section is a little to the side so as to 

 avoid passing through the ocular cleft, c, 

 the cuticle, where it becomes later the 

 cornea; I, the lens; op, optic nerve 

 formed by the pedicle of the primary optic 

 vesicle ; vp, primary medullary cavity or 

 optic vesicle ; p, the pigment-layer of the 

 choroid coat of the outer wall ; r, the 

 inner wall forming the retina ; vs, secon- 

 dary optic vesicle containing the rudiment 

 of the vitreous humour. 



cavity in which the vitreous humour is formed ; and, the forepart of the optic nerve 

 participating in the imagination, it is by this means that the central artery of the 

 retina is introduced into the nerve and the eyeball, being, as it were, folded within 

 them. The deficiency in the wall of the cup of the secondary vesicle inferiorly is 



Fig. 492. 



Fig. 492. TRANSVERSE VERTICAL SECTION OF THE 

 EYEBALL OP A HUMAN EMBRYO OP FOUR WEEKS 

 (from Kolliker). *- 



The anterior half of the section is represented. 

 pr, the remains of the cavity of the primary optic 

 vesicle ; p, the inner part of the outer layer, form- 

 ing the choroidal pigment; r, the thickened inner 

 part giving rise to the columnar and other struc- 

 tures of the retina ; v, the commencing vitreous 

 hiimour within the secondary optic vesicle ; v', the 

 ocular cleft through which the loop of the central 

 blood-vessel, a, projects from below ; l t the lens 

 with a central cavity. 



gradually filled up by the growing downwards of the edges, until only a cleft is left, 

 which is discernible for a considerable time, and has been named the ocular cleft. The 

 history of this cleft is of interest in connection with congenital fissure of the iris 

 (coloboma iridis) and the accompanying condition of the choroid membrane. Some 

 difference of opinion exists with regard to the subsequent history of the walls of the 

 secondary vesicle, but the opinion of Kolliker appears to be well founded, that the 

 invaginated layer forms the retina, and the outer part the pigmentary epithelium 

 of the choroid. Thus the elements of Jacob's membrane and the hexagonal cells of 

 the choroid may be regarded as originally continuous, forming together the epithelial 

 lining of the cavity of the primary vesicle ; and the development of nervous tissue 

 underneath Jacob's membrane, while none exist beneath the choroidal layer, is a 

 circumstance which may be looked upon as analogous to the absence of nervous tissue 

 from various parts of the walls of the cerebral vesicles. The sclerotic coat and 

 cornea are formed from the surrounding tissue external to the parts of the eye which 

 they enclose ; and, according to Kolliker, the vascular part of the choroid is of later 

 formation. Still later, in the second month of foetal life, the iris begins to be formed 



