THE ORGANS OP THE OUTER GERM-LAYER. 



469 



tum, tte epidermis, by means of a solid epithelial cord (1st). Upon 

 "being constricted ofi" the lens-vesicle naturally pushes the adjacent 

 lateral wall of the optic vesicle before it and folds the latter in 

 against the median wall. 



At the same time with the development of the lens, the primary 

 optic vesicle is also invaginated from below along a line which 

 stretches from the epidermis to the attachment of the stalk of the 

 optic vesicle, and is even continued along the latter for some distance 

 (fig. 265 aus). A loop of a blood-vessel from the enveloping 

 connective tissue, embedded in soft, gelatinous substance (gl), here 

 grows against the lower surface of the primary optic vesicle and its 

 stalk, and pushes up before it the 

 dower wall. 



In consequence of the two invagina- 

 tions the optic vesicle acquires the 

 form of a beaker or cup, the foot of 

 which is represented by its stalk {Sn). 

 But the optic cup, as we can from this 

 time forward designate the structure; 

 exhibits two peculiarities. First, it 

 lias, as it were, a defect (fig. 265 atcs) 

 in its lower wall ; for there runs along 

 "the latter from the margin of the 

 broad opening which embraces the 

 lens (l) to the attachment of the stalk 

 (Sn) a fissure (aus), which is caused by 

 the development of the vitreous body 

 (gl) and bears the name foetal optic 

 fissure [or choroid Jissure]. At first 

 it is rather wide, but then becomes narrower and narrower by the 

 approximation of its edges and finally closed altogether. Secondly, 

 the optic cup, like the toy called the cup of Tantalus, is provided 

 with double walls, which are continuous with each other along the 

 ■edge of the front opening and also along the fissure. They will 

 henceforth be designated as inner (figs. 264 B and 265 ib) and outer 

 (ab) layers; the former is the invaginated, the latter the unin- 

 vaginated part of the primary optic vesicle. 



At the beginning of the infolding the two layers are separated by 

 a broad space (h), which leads into the third ventricle through the 

 stalk of the vesicle (Sn) ; but afterwards the space becomes reduced 

 proportionally to the increase in the size of the vitreous body. 



ail, 



fig. 265.— Plastic representation of 

 the optic cup witli lens and 

 vitreous body. 



Outer wall of the cup ; ib, its 

 inner wall ; h, cavity between 

 the two walls, which later dis- 

 appears entirely ; Sn, fundament 

 of the optic nerve. (Stalk of the 

 optic vesicle with a furrow on 

 its lower sui-face.) aus, Optic 

 [choroid] fissure ; gl, vitreous 

 body ; I, lens. 



