THE EMBRYO. 



1181 



of the vesicle is 

 and the original 



almost obliterated 

 vesicle converted 



'Forebrain. 



in Figs. 729 and 730, so that the cavity 

 by the folding back of its anterior half, 

 into a cup, the optic cup, in which the 

 involuted epiblastic layer, the rudiment 

 of the lens, is received (Fig. 730) ; at 

 the same time the proximal part of the 

 vesicle becomes elongated and narrowed 

 into a hollow stalk, the optic stalk. 

 This cup-shaped cavity consists there- 

 fore of two layers : one, the outer, orig- 

 inally the posterior half of the primi- 

 tive ocular vesicle, is thin, and even- 

 tually forms the pigmented layer of the 

 retina ; l the other layer, the inner, orig- 

 inally the anterior or more prominent 

 half, which has become folded back, is 

 much thicker, and is converted into the 

 nervous layers of the retina (Figs. 

 730 and 732). Between the two is 



the remnant of the cavity of the original primary optic vesicle, which finally 

 becomes obliterated by the union of its two layers. When . the retina is estab- 

 lished, the optic nerve-fibres originate from its cells and grow backward toward 

 the brain, along the optic stalk, and thus convert it into a solid optic nerve. The 

 nerve-fibres become ultimately connected with the mesencephalon, a relationship 



Pigmented layer 

 /of retina. 



/Epiblast. 



Lens rudiment. 

 Nervous part 

 of retina. 



Optic stalk. 

 FIG. 730. Transverse section of head of chick embryo 



of fifty -two hours' 

 d' Embryologie.) 



incubation. (From Duval's Atlas 



FIG. 731. Diagram of development of the lens. A, 

 B, c. Different stages of development. 1. Epidermic 

 layer. 2. Thickening of this layer. 3. Crystalline 

 depression. 4. Primitive ocular vesicle, its anterior 

 part pushed back by the crystalline depression. 5. 

 Posterior part of the primitive ocular vesicle, form- 

 ing the external layer of the secondary ocular vesi- 

 cle. 6. Point of separation between the lens and the 

 epidermic layer. 7. Cavity of the secondary ocular 

 vesicle, occupied by the vitreous. 



FIG. 732. Diagrammatic sketch of a vertical 

 longitudinal section through the eyeball of a 

 human fostus of four weeks. (After Kolliker.) 

 Magnified 100 diameters. The section is a little 

 to the side, so as to avoid passing through the 

 ocular cleft, c. The cuticle, where it becomes 

 later the epithelium of the cornea. I. The lens. 

 op. Optic nerve formed by the pedicle of the 

 primary optic vesicle, vp. Primary medullary 

 cavity of the optic vesicle, p. The pigment layer 

 of the retina, r. The inner wall forming the 

 nervous layers of the retina, rs. Secondary optic 

 vesicle containing rudiment of the vitreous 

 humor. 



which is permanently maintained. The mouth of the optic cup overlaps the 

 equator of the lens as far as the future aperture of the pupil. In this region 

 the inner or retinal layer of the cup does not become differentiated into nervous 

 elements, but remains as a single layer of columnar cells, which becomes applied 

 to the cells of the pigmented layer, and the conjoined strata form the pars ciliaris 

 pars iridica retinae of the adult (Fig. 734). As development proceeds the optic 

 cup increases in size, and thus a space is formed between it and the rudimentary 

 lens ; this is the secondary optic vesicle, and in it the vitreous humor is developed 

 (Figs, 731, c, and 732). The folding in of the primary optic vesicle to produce 



1 This layer was formerly described as belonging to the choroid, but developmentally it is seen 

 to be a part of the retina. 



