83 



DEVELOPMENT OF THE EYE. 



this epithelium. Into this homogeneous layer mesoblast cells pass from the margin, 

 greatly thickening it and producing eventually the regular layers of fibrous tissue, 

 which are characteristic of the cornea. No cells pass into the most anterior or 

 into the most posterior stratum, which remain homogeneous (anterior and posterior 



Fig. 103. HORIZONTAL SECTION 



THROUGH THE EYE OF AN EMBRYO 

 RABBIT OF 18 DAYS. 3 ^. (Kol- 



liker.) 



o, optic nerve ; p, hexagonal pig- 

 ment layer ; r, retina ; re, ciliary 

 part of the retina ; p', forepart of the 

 optic cup (rudiment of the iris pig- 

 ment) ; g, vitreous, shrunk away from 

 the retina, except where the vessels 

 from the arteria centralis retinas enter 

 it ; i, iris ; mp, membrana pupillaris ; 

 c, cornea with epithelium e ; pp, pa, 

 palpebrae ; I, lens ; I', lens epithe- 

 l' limn ; /, sclerotic ; m, recti muscles. 



homogeneous lamellae of Bow- 

 man). The epithelium of the 

 posterior homogeneous lamella, 

 or membrane of Descemet, is 

 derived from mesoblast cells 

 which grow in like the cor- 

 neal corpuscles from the mar- 

 gin and spread themselves 

 over the posterior surface of 

 the cornea, thus separating 

 this from the iris and anterior 

 surface of the lens. For a 



long while, however, there is no anterior chamber; this eventually appears as a 

 < Isft-like space between the cornea and the structures immediately behind it. 



In mammals, all the above stages of formation have not been described. A 

 complete layer of mesoblasb is early visible lying between the corneal epiblast and 

 tne lens epiblast, and continuous around the margin of the lens with the mesoblast 

 of the vitreous chamber. In this mesoblast a cleft makes its appearance, separating 

 it into two parts, one of which adheres to the corneal epiblast, where it forms the 

 substance of the cornea, the other to the lens capsule forming the pupillary membrane. 

 This cleft is the rudiment of the anterior chamber. It does not become actually 

 distended with fluid until a short time before birth (Kolliker). 



The sclerotic is formed entirely from mesoblast around the optic cup, probably 

 continuous with that which forms the cornea, although it, is only later that the 

 cornea and sclerotic come to be completely amalgamated. 



The choroid coat is formed from the mesoblast which is immediately in contact 

 with the outer layer of the optic cup, and the forward growth of the middle tunic 

 closely follows that of the margin of the cup. The latter ceases at first at the 

 margin of the lens, but subsequently grows forwards over the front of the lens as a 

 t,hin double layer, which is closely covered externally with a continuation of the 

 choroidal mesoblast. This is the iris, over the back of which both the layers of the 

 cup-margin eventually acquire pigment and remain permanently as the uvea. The 

 ciliary body is formed by a kind of hypertrophy of the optic cup, which developes 

 radial folds, enclosing thin portions of mesoblastic choroidal tissue, in which, as in 

 the rest of the choroid, numerous blood-vessels and branched pigment-cells become 

 formed. 



