22 



explanation is defended most persistently by Wolfrum, who main- 

 tains moreover that the hyaloid canal is not found in the eyes of 

 adult animals, except in connection with remnants of the hyaloid 

 artery. The discussion of the latter statement we reserve until 

 later. For the embryonic eye, however, we can not accept 

 Wolf rum's explanation. 



Equatorial sections prepared for the purpose of ascertaining 

 the precise time of the first appearance of the hyaloid canal show 

 that it is formed in embryos between 130 and 150 mm length. From 

 the first it shows its characteristic funnel-like shape, its size consid- 

 erably exceeding the diameter of the blood vessel (Figure 20). The 

 latter most frequently appears in the center of the canal and shows 

 no relation to its walls (Figure 18). In a number of longitudinal 

 sections of the eyes of embryos ranging from 150 to 180 mm length, 

 the slowly degenerating hyaloid system consists of the main trunk, 

 the hyaloid artery, and several branches surrounding it in a manner 

 not unlike the supporting framework of a tent. At this time there 

 is observed likewise an increased growth of the vitreous fibers from 

 the ciliary region of the retina. A portion of these fibers, as was 

 shown above, takes its course toward the lens and then turns inward 

 toward the centre of the eye. It is this portion which is so prom- 

 inent in figure 18. Now it seems to us very probable that this 

 peculiar direction is given to the fibers by the branches of the blood 

 vessels. In many sections, parts of the latter are almost invariably 

 found in a plane parallel to the vitreous fiber bundles (Figure 21). 

 But whereas the blood vessels finally disappear, the direction given 

 to the fibers remains, thus determining the outline and shape of the 

 hyaloid canal of which they form the membrane-like wall. 



This denser formation of the vitreous fibers, descending in a cir- 

 cular band from the ciliary retina encloses a large portion of the 

 original vitreous body which forms the fibrous contents of the hya- 

 loid canal. This irregular fibrous mass may, therefore, be regarded 

 as the remnant of the central portion of the vitreous body of the 

 preceding period of development. A similar explanation is offered 

 for the irregular fibrous tissue between the two heavier layers of 

 vitreous fibers in the lateral parts of the eye. Thus the hyaloid 

 canal is not a canal in the generally accepted meaning of the word, 

 yet it possesses such a definite structure, and it is so sharply marked 

 off from the rest of the vitreous body that it deserves a special 



