20 



and extends not to the lens, but to the ciliary portion of the retina, 

 which gives rise to the vitreous fibers. Its vast size, compared with 

 the diameter of the hyaloid artery, seems to dispose at once of the 

 theory that the hyaloid canal simply represents the portion of the 

 vitreous body formerly occupied by that blood vessel. Its walls are 

 formed by the vitreous fibers which in many sections have all the 

 appearances of a real membrane (Figure 18). This impression is 

 heightened by closer study. Still we hesitate to describe this por- 

 tion as a special membrane. We rather incline to the belief that the 

 membranous appearance is owing to the thickness of the section, 

 which in this case is not less than 400 microns. The fibrous nature 

 of this structure seems to be quite plainly shown in the upper left- 

 hand portions of figure 18. It is not improbable, however, that the 

 fibers are held together by the thickened fluids of the vitreous body, 

 giving it the firm membranous appearance it possesses. This portion 

 of the vitreous body, although generally called the hyaloid canal, is 

 in reality not a canal. It is not an open tube without structural 

 elements, because its interior is filled with an irregular mass of 

 tissue, not unlike that of the central portions of the vitreous body 

 enclosed between the outer and inner fiber bundles already described 

 and illustrated in figure 18. 



To complete the description of the eye at this period we call 

 attention to another new structure which has appeared in the mean 

 time, the zonula ciliaris (Figure 15). The sketch is prepared from 

 a section of the left eye of the same embryo which furnished the 

 material for the illustrations on plate. The zonula ciliaris consists 

 of a number of rather strong fibers between the pars ciliaris retinae 

 and the lens. They arise, as is clearly shown in many sections, from 

 the region of the retina, that produces the vitreous fibers, only 

 slightly anterior to the latter. Ketina, vitreous body, and zonula 

 ciliaris, therefore, are genetically most intimately related. 



Keeping in mind the structure of the eye at this rather advanced 

 stage of embryonic life, we may now attempt to trace the develop- 

 ment of the various parts of the vitreous body up to this time. In 

 embryos of 80 mm length, the hyaloid vascular system consists of 

 the main trunk and a number of branches, some of which surround 

 the posterior half of the lens, while the rest form a complex system 

 in the outer portion of the vitreous body near the retina. The 

 entire system is only a temporary formation, whose function is to 

 furnish nourishment to the rapidly developing lens, and no doubt. 



