376 



THE PERIPHERAL NERVOUS SYSTEM 



I 



Both the inner and outer layers of the optic cup are continued into the optic 

 stalk, as seen in Fig. 357. This is due to the invagination of the ventral wall of 

 the optic stalk and the formation in it of the chorioid fissure when the optic vesicle 

 is transformed into the optic cup (Fig. 356). Into the chorioid fissure grows the 

 central artery of the retina, and carries with it into the posterior cavity of the 

 eye a small amount of mesenchyme, as seen in the eye of a 1 2 mm. embryo (Fig. 

 358). Branches from this vessel extend to the posterior surface of the lens and 

 supply it with nutriment for its growth. At a later stage the chorioid fissure 

 closes, so that the distal rim of the optic cup forms a complete circle. 



{Epithelial layeroflens 



f Pigment layer of the retina. 



Nervous layer of reiina. 



Chorioid fissure 



FIG. 358. Transverse section passing through the optic cup at the level of the chorioid fissure. 

 The central artery of the retina is seen entering the fissure and sending a branch to the proximal surface 

 of the lens; from a 12.5 mm. embryo. X 105. 



If the chorioid fissure fails to close, the optic cup remains open at one point and this 

 results in the defective development of the iris, ciliary body and chorioid layer. Such defects 

 are known as coloboma. 



It was formerly supposed that the development of the lens vesicle caused the formation of 

 the optic cup by pushing in its distal wall. It has been shown by W. H. Lewis that this is not 

 the case, for if an anlage of the optic vesicle from an amphibian embryo is transplanted to some 

 other part of the embryo, it will not only develop into an optic cup, but the ectoderm over 

 it will differentiate a lens vesicle. 



The lens vesicle in its early development from the ectoderm has been de- 

 scribed. Its proximal wall is much thickened in 10 mm. embryos and these cells 

 form the lens fibers (Fig. 357). A few cells early separated off from the wall of 



