1182 EMBR YOL OGY. 



the optic cup not only takes place in front, at its most prominent part, opposite 

 the lens, but also along its postero-inferior aspect, where a cleft or fissure is 

 formed, the choroidal fissure, through which the mesoblast extends to form the 

 vitreous humor. This gap or cleft is continued for some distance into the stalk of 

 the optic vesicle, and thus allows a process of the mesoblast to extend down the 

 stalk to form the arteria centralis retinae arid its accompanying vein (Fig. 733). 

 After a time the gap or fissure becomes closed, by a coalescence of its margins, but 

 the line of union remains apparent for a considerable period. 



The lens is at first a thickening of the epiblast, then a depression or involution 

 takes place, thus forming an open follicle, the margins of which gradually 



Prosencephalon. 



Margin of 

 optic cup. 



Thalamen- 

 ecphalon.~jji jjj f ..Choroidal 



.Optic stalk. 



^Central artery 

 of retina. 



FIG. 733. Optic cup and choroidal fissure seen from below, from a human embryo of about four weeks. 



(Kollmann.) 



approach each other and coalesce, forming a cavity, the lens vesicle, enclosed by 

 epiblastic cells (Fig. 731, B c). At the point of involution the external layer of 

 epiblast separates from the lens and passes freely over the surface, so that the lens 

 becomes disconnected from the general epiblast, and recedes into the ocular cup, 

 while the cuticular layer covering it is developed into the corneal epithelium. The 

 cells forming the posterior or inner wall of the lens vesicle rapidly increase in 

 size, becoming elongated and developed into the lens-fibres, and, filling up the 

 cavity, convert it into a solid body. The cells on the anterior wall retain their 

 cellular character, arid form the anterior lens epithelium of the adult. The 

 secondary optic vesicle, or space between the lens and the hollow of the optic cup 

 (Figs. 731, 7, and 734), contains a quantity of mesoblastic tissue continuous with 

 the general mesoblast through the choroidal fissure. This tissue becomes con- 

 verted into the vitreous humor, and surrounds the lens with a vascular membrane, 

 the vascular capsule of the lens. From the central artery of the retina several 

 branches are prolonged forward through the vitreous body to the capsule of the 

 lens, but by the sixth month these have all undergone atrophy except one, which 

 persists till the ninth month as the arteria liyaloidea. It disappears, however, 

 before birth, and its position is indicated in the adult by the canalis hyaloideus of 

 Stilling. The front part of the vascular capsule of the crystalline lens forms the 

 membrana pupillaris, and also attaches the iris to the capsule of the lens. It dis- 

 appears about the seventh month. The sclera, cornea, and choroid are developed 

 from the mesoblast surrounding the optic vesicle. 



The eyelids are formed at the end of the third month, as small cutaneous folds 

 (Fig. 734), which come together and unite in front of the globe and cornea. This 

 union is broken up and the eyelids separate before the end of foetal life. 



The lachrymal sac and nasal duct appear to result from a thickening of the 



