RETINA AND OPTIC STALK 141 



lamina before it, so that a part projects into the space between the layers of the optic cup. From 

 this a protoplasmic process is developed, which, elongating, becomes transversely striated and 

 converted into the cuticular rod. 



The optic stalk is, as we have already seen, hollow. Its lower wall is thicker 

 than its upper and is invaginated by the choroidal fissure at its ocular end. The 

 fissure soon closes, and the artery, entering the optic cup within the stalk, is 

 enclosed. The ventricular cavity is obliterated by the middle of the second 

 month, but the epithelial cells retain for some time longer their radial disposition. 

 This soon becomes lost, and the stalk becomes converted into a glial network in 



^- pp 



lens * k\va 



FIG. 183. SECTION OF THE DEVELOPING EYE OF TROUT. (Szily.) 



lens, lens-vesicle not yet closed ; ret, inner layer, pp, outer layer of optic cup ; p.v., primitive 

 vitreous; a, protoplasmic connexions between the cells of the outer and inner walls of the optic cup. 



which the nerve-fibres appear at the end of the second month (His). The nerve- 

 fibres begin in the retina and grow along the stalk towards the brain, extending 

 into its thickened invaginated lower wall. The point where this is continuous 

 with the retina at the centre of the optic cup becomes the optic disc. The optic 

 chiasma is formed by fibres crossing in the posterior boundary of the optic recess, 

 and the optic tract is a new formation by which the eye is secondarily connected 

 with the optic thalamus and mid-brain. 



Development of the vitreous body and lens capsule. 1 The formed 

 elements of the vitreous body and the zonule of Zinn are to be regarded as a special 



1 The following account is founded on the observations of Toruatola, Rabl, Addario, Van Pee, 

 Lenhossek, Kolliker, and Szily. My own observations, on which the actual description is based, have 

 been made on rabbit material. T. H. B. 



