EYE. 



181 



EYE. 



In contrast to the eyes of Invertebrates, which arise by a 

 differentiation of the cells of the epiblast (hypodermis), 1 the 

 sensitive elements of the Vertebrate eye are formed from a paired 

 outgrowth of the primary vesicle of the fore-brain (Figs. 150, 15lA, 

 ABl], as already mentioned on p. 132. 



This outgrowth is spoken of as the primary optic vesicle, 

 and as it grows outwards towards the outer skin of the embryo, 



A 



t t 



B 



BSP 



FIG. 150. DIAGRAMS SHOWING THE MODE OF FORMATION OF THE EYE IN 

 INVERTEBRATES (A) AND VERTEBRATES (B). 



Ep, Ep, epiblast, which is invaginated iu A to form the retina (It), and in B to form 

 the medullary groove, its cavity (l r ) representing the third ventricle of the 

 brain ; in both figures ft indicate the point where the external epiblast becomes 

 continuous with the invaginated portion ; the arrow in Fig. A shows the direc- 

 tion in which the rays of light pass, and one can imagine the rays entering in 

 the same manner in Fig. B, and passing in the direction of the curved arrows : 

 they would thus fall upon the shaded cells B&P on the walls of the ventricle, 

 that is of the primary optic vesicle, which give rise later to the retina ; L, the 

 cells of epiblast, out of which the lens is formed later ; Jnf, infunJibulum ; 

 N, optic nerve. 



the portion which connects it with the brain becomes constricted, 

 and by degrees loses its cavity, giving rise to a solid cord, from 

 which the optic nerve is formed. 



At the point where the vesicle touches the epiblast, the latter 

 becomes thickened, and gives rise to a mass of cells which is at 

 first hollow, but which becomes compact later. This thickened 

 portion of the epiblast then becomes driven inwards, carrying before 



1 On closer consideration, this distinction does not appear to be an essential one, 

 for the outgrowths of the primary wall of the brain arise in the embryo at a time when 

 the medullary groove is still open to the exterior, and when the cells comprising it con- 

 sist simply of a thickened portion of the epiblast. When the primary optic vesicle 

 conies into apposition with the epiblast, and the lens and secondary optic vesicle are 

 formed, the relations of the parts are different, the sensitive (neuro-epithelial) 

 elements of the retina being turned away from the light. (Comp. Figs. 150 and 

 151A and E, and p. 190.) 



