THE ORGANS OF THE OUTER GERM-LAYER. 



469 



turn, the epidermis, by means of a solid epithelial cord (1st). Upon 

 being constricted off 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 



lower 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 (Sri). 



But the optic cup, as we can from this 



time forward designate the structure* 



exhibits two peculiarities. First, it 



has, as it were, a defect (fig. 265 aus) 



in its lower wall ; for there runs along 



the latter from the margin of the 



broad opening which embraces the 



lens (I) to the attachment of the stalk 



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

 the development of the vitreous body 

 (gl) and hears the name foetal optic 

 fissure [or choroid fissure]. 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. 



Pig. 265. Plastic representation of 

 the optic cup with lens and 

 vitreous body. 



ab, Outer wall of the cup; ib, its 

 inner wall ; k, 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 surface.) aut, Optic 

 [choroid] fissure ; gl, vitreous 

 body ; I, lens. 



