DEVELOPMENT OF THE EYE. 



1481 



Brain-vesicle 



Uninvaginated 

 wall 



Invaginated wall 

 Optic stalk 



Optic vesicle 

 Lens-pit 



Lens-pit shows as depressed area of thickened 

 ectoblast ; anterior wall of optic vesicle beginning 

 to be invaginated ; optic stalk narrowing. X 30. 



uninvaginated portion of the wall, on the contrary, gradually becomes thinner, until it is repre- 

 sented by a single layer of cubical cells. These soon assume a dark color in consequence 

 of the appearance within their protoplasm of fine pigment particles. From this wall, there- 

 fore, the layer of pigmented cells composing the outermost stratum of the retina is developed, 

 whilst from the rapidly augmenting layers of the inner wall, the essential nervous elements 

 of the retina, together with the supporting neurogliar tissue, are formed. 



The invagination of the optic vesicle is not confined to its outer wall, but also affects its 

 lower wall, in consequence of which a groove, the fietal ocular cleft, appears in this position 

 (Fig. 1240). This is continued backward to and along the under surface of the optic stalk, in 

 the form of a furrow. By means of this slit a com- 

 munication is established between the cavity of the FIG. 1238. 

 secondary optic vesicle and the centre of the optic 

 stalk, and through it blood-vessels from the sur- 

 rounding mesoblast gain entrance to the interior of 

 the nerve and the eyeball. The walls of this fcetal 

 cleft gradually approximate and become fused. The 

 imprisoned vessel, the hyaloid artery, later gives 

 rise to the arteria centralis retinas. The vitreous 

 body has been usually considered as a derivative 

 exclusively of mesoblastic tissue which entered the 

 eye in company with the blood-vessels. According 

 to the investigations of Schon, Kolliker, Wolfrum 

 and others, however, this view is inadequate, since at 

 least the anterior or ciliary portion of the vitreous 

 is a product of the cells of the inner wall of the 

 secondary optic vesicle. The choroid and the 

 sclera are differentiated from the mesoblast, which 

 surrounds the eyeball. 



Development of the Lens. Soon after the iso- 

 lation of the primitive lens- vesicle from the surface ectoblast, the cells in the posterior wall 

 begin to proliferate actively, while those on the anterior wall are reduced to a single layer. 

 The latter persists as the lining epithelium of the adult lens-capsule. By the growth of the 

 cells of the posterior wall and their elongation into lens-fibres, the hollow vesicle is gradually 

 converted into a solid mass of lens-substance, the fibres extending forward until they come in 

 contact with the anterior wall. Subsequently the growth of the lens proceeds by the application 

 of additional layers of fibres to the surface of the primary nucleus, the new fibres developing 



from the cells lining the anterior capsule. Their con- 



FIG. 1239. version takes place at the equator of the lens, where 



the nuclei of the elongating lens-fibre are arranged in 

 a convex line known as the nuclear zone (Fig. 1228). 

 The capsule of the lens appears very early, 

 even before the closure of the lens-vesicle, and long 

 before the appearance of blood-vessels around the 

 lens. It forms a sharp boundary line, at first along 

 the posterior border, which gradually thickens and 

 finally surrounds the entire lens. The capsule is to 

 be considered as a secretion product of the lens-cells. 

 The rapid early growth of the young lens 

 requires an adequate blood supply. This is insured 

 by the development of a vascular net-work, the 

 tunica vasculosa lentis, which completely surrounds 

 the lens from the second month until the close of 

 fcetal life, when this temporary membrane is ab- 

 sorbed. The chief supply of this vascular net-work 

 is derived from the vessels of the vitreous, which, 

 as already noted, enter the eye through the cleft in 

 the optic nerve. Passing forward through the canal of Cloquet in the centre of the vitreous 

 cavity, the chief vessel, the hyaloid artery, reaches the posterior pole of the lens, when it divides 

 into numerous branches. These branches pass around the equator of the lens onto the anterior 

 surface, where, joined by vessels from the mesoblastic tissue which is to constitute the future 

 iris and ciliary body, they proceed to the centre of the pupil and break up into their terminal 

 loops. The portion of the net-work covering the pupillary area is called the membrana pupil- 

 laris, whilst the remainder is known as the membrana capsularis. This vascular sheet is usually 

 entirely absorbed before birth, but occasionally portions of it may be seen persisting in the 

 form of fine threads in the pupillary space, or on the posterior pole of the lens. The retention 

 of such strands is sometimes associated with the persistence of portions of the hyaloid artery. 







,r Ectoblast 



Outer layer 

 Lip of optic cup 

 Inner layer 

 Anterior wall 

 Optic stalk 



Posterior wall 

 of lens-sac 



Lens-sac closed ; outer and inner layers of sec- 

 ondary optic vesicle now almost in contact. X 30. 



