VITKEOUS, SCLEROTIC, AND CHOROID 



143 



<tp 



fibrils from the ciliary epithelium form the greater part of the formed portion 

 of the vitreous, the so-called hyaloid membrane, and zonule of Zinn. 



The syncytial meshwork in front of the lens has a different fate. It becomes 

 invaded by large numbers of mesenchyme-cells, and is converted into a thick 

 cellular plate between the lens and the surface epithelium. At this stage a delicate 

 refractile membrane appears round the lens, formed apparently from the mesh- 

 vrork, and this, together with the mesenchymatous layer behind the lens, and 

 a delicate lamella of mesenchyme in front of it separated off at a later stage from 

 the anterior mass of mesoderm becomes the lens-capsule. 



The membrane thus defined becomes the tunica vascidasa of the later stages of foetal develop- 

 ment. The portion behind the equator of the lens is supplied by the hyaloid artery, and the 

 part in front of the equator by the 

 anterior ciliary arteries. Shortly 

 before birth the vessels disappear : 

 the hyaloid artery is obliterated, 

 but its track in the vitreous per- 

 sists as the canal of Stilling. The 

 fore- part of the capsule is named 

 the pupillary membrane. ' 



Development of the 

 protective and vascular 

 coats ; the iris and 

 aqueous chamber. The 



optic cup is surrounded by a 



specially vascular mass of 



mesenchyme, and, as we have 



seen, this extends into the 



space between the lens and 



the surface epithelium. It 



here forms a thick cellular 



layer, which is the rudiment 



in great part of the cornea, 



while the surface ectoderm 



over it forms the corneal 



epithelium. The cornea is at 



first identical in structure 



with the primitive sclerotic, 



with which it is, of course, 



continuous. Later it under- 



goes histological changes 



which cause it to become 



transparent. The mesen- 



chyme surrounding the optic 



cup becomes differentiated 



into two layers an outer 



more condensed stratum which forms the sclerotic coat, and an inner, looser stratum, 



enclosing many vessels, which becomes the choroid coat. This is thickened 



near the margin of the cup to form the ciliary body, and its surface becomes 



radially folded over the thickening to give rise to the ciliary processes. On its 



outer side the ciliary muscle is laid down. 



The aqueous chamber is formed by a separation of the mass of mesenchyme 

 between lens and surface into two lamellae a thick anterior layer which becomes 



1 For an exhaustive account of the development of the arteries of the mammalian eye, see Fuchs, 

 Anat. Hefte xxviii. 1905. 



FlG. 185. HOBIZONTAL SECTION THROUGH 

 EMBBYO-RABBIT OP EIGHTEEN DAYS. S T 



AN 



o, optic nerve ; 



THE EYE OF 



1 (Kolliker.) 



ve ; p, hexagonal pigment-layer ; r, retina ; 

 re, ciliary part of the retina ; p', fore-part of the optic cup 

 (rudiment of the iris-pigment) ; g, vitreous, shrunk away from 

 the retina, except where the vessels from the arteria centralis 

 retinae enter it ; i, iris ; ?np, membrana pupillaris ; c, cornea 

 with epithelium e ; pp,pa, palpebrse ; I, lens ; I', lens-epithelium ; 

 /, sclerotic ; m, recti muscles. The formation of the aqueous 

 chamber is just beginning, and is seen as a cleft in front of 

 the iris on each side. 



