3 So 



THE PERIPHERAL NERVOUS SYSTEM 



peripheral fibers overlap the ends of the' shorter, central fibers. By an 

 intricate but orderly arrangement of fibers these sutures are later trans- 

 formed into lens-stars of three, and finally of six or nine rays (Fig. 379). 

 The structureless capsule of the lens is probably derived from the lens cells. 

 The lens, at first somewhat triangular in cross section, becomes nearly 

 spherical at three months (Fig. 379) 



Anterior epithelium 

 of corned 



Raphe between 

 the. fused lids 



Posterior epithelium 

 of cornea 



wm 



li 



Cornea 



Epithelium 

 of lens 



. 



Pars iridica 

 \v! >. re/ince 



i'.Vi, 



Pigment layer 

 of retina 



Pars oplica 



FIG. 379. Section through the distal half of the eyeball and eyelids of a 65 mm. human fetus. 



X 35- 



The origin of the vitreous body, whether ectodermal or mesodermal, 

 has long been in doubt. Modern evidence apparently points to its deri- 

 vation from both sources. 



It is certain that vitreous tissue is formed before mesenchyma is present in the cavity 

 of the optic cup. Szily (1908) regards this primitive vitreous body as a derivative of both 

 retinal and lens cells, it forming a non-cellular network of cytoplasmic processes which are 

 continuous with the cells of the lens and retina. With the ingrowth of the central artery 

 of the retina, from which the artery of the lens passes to the proximal surface of the lens 

 and branches on it, a certain amount of mesenchymal tissue invades the optic cup, and this 

 tissue probably contributes to the development of the vitreous body (Fig. 377). 



