114 THE GENESIS OF THE VERTEBRATE EYE 



contributions. Most of the definitive or secondary vitreous is secreted 

 by the retina during the growth of the eye, the primary vitreous coming 

 to form a slender cone with its base on the back of the lens and its apex 

 at the head of the optic nerve (Fig. 43). The final disappearance, from 

 this primary vitreous, of the last remnants of the hyaloid circulation 

 leaves it in the form of a conical tube (filled with vitreous thinner than 



Fig. 43 — Formation of the vitreous. 



a, diagrammatic section of young optic cup showing vessels of the hyaloid system embedded 

 in the primary vitreous, consisting of mesodermal fibers and cells (which invaded the cup 

 along with the hyaloid artery ha), together with fibrils secreted by the ectoderm of the cup, 

 lens, and surface. 



b, diagrammatic section of fetal optic cup in which atrophy of the vasa hyaloidea propria 

 (c/. Fig. 42b) has clarified the peripheral vitreous, to which has now been added much 

 secondary vitreous (vertical hatching) secreted by the sensory retina. The persisting tunica 

 vasculosa lentis and the trunk of the hyaloid artery ha are embedded in a cone of primary 

 vitreous. 



c, the definitive situation (c/. Fig. 3, p, 7): the canal of Cloquet represents the remnants 

 of the primary vitreous, stretched to a slender column by the growth of the eye (diagonal 

 hatching). The secondary vitreous (vertical hatching) nearly fills the globe. The tertiary 

 vitreous (horizontal hatching) is constituted by the fibers of the zonule, secreted lastly by 

 the non-sensory retina. The optic-nerve portion of the hyaloid artery alone persists, as the 

 central retinal artery era, and has given off new branches into the retinal tissue. 



the secondary kind) , the canal of Cloquet. This canal runs through the 

 vitreous from disc to lens in the adult, with a considerable sag along 

 its course caused by gravity and time (see Fig. 3, p. 7). 



The Vascular and Fibrous Tunics — As soon as pigment granules 

 appear in the outer layer of the finished optic cup, a network of capil- 

 laries — the future choriocapillaris — is formed in the mesoderm against 

 the pigment epithelium. Larger vessels developing outside of these, and 



