THE EYE. 



123 



The first appearance of the eye consists in the protrusion or evagination from 

 the medullary wall of the thalamencephalon, or inter-brain, of a vesicle, called the 

 /,////<>>/' ocular This is at first an open cavity communicating by a hollow 



stalk with the general cavity of the cerebral vesicle. As the primitive ocular vesi- 

 cle is prolonged forward, it meets the epidermic layer of the epiblast, which at the 

 point of contact becomes thickened, and then forms a depression which gradually 

 encroaches on the most prominent part of the primitive ocular vesicle, which in its 

 turn appears to recede before it, so as to become at first depressed and then inverted 

 in the manner indicated by the annexed figure (Fig. 94, A), so that the cavity is 

 finally almost obliterated by the folding back of its anterior half, and the original 

 sac converted into a cup-shaped cavity, the ocular cup, in which the involuted 

 epiblastic layer, the rudiment of the lens, is received (Fig- 94, B). This cup- 

 shaped cavity consists therefore of two layers : one, the outer, originally the 

 posterior half of the primitive ocular vesicle, is thin, and eventually forms the 

 pigmental layer of the retina : l the other layer, the inner, originally the anterior 

 or more prominent half, which has become folded back, and is much thicker, 

 is converted into the nervous layers of the retina. Between the two are the 

 remains of the cavity of the original primary vesicle, which finally becomes 

 obliterated by the union of its two layers. The optic nerve fibres originate from 

 the cells of the ganglionic layer of the retina, which thus correspond to the cells ot 

 the posterior root ganglia of the spinal nerves. From these cells the fibres grow 

 toward the brain, choosing the optic stalk as a path along which to grow, the stalk 

 thus becoming gradually replaced by the optic nerve. As development proceeds the 

 cup-shaped cavity or ocular cup increases in size, and thus a space is formed between 

 it and the rudimentary lens which it contains ; this is the secondary ocular vesicle, 

 and in it the vitreous humor is developed (Fig. 94, c). The folding in of the primary 

 optic vesicle to produce the optic cup proceeds from above downward, and grad- 

 ually surrounds the lens, but leaves an aperture or fissure below, the choroidal 

 Ussure or oculir cleft, through which vascular elements, within the vesicle and 

 derived from the mesoblast. retain their con- 

 nection with the rest of the mesoblast. This 

 gap or clefc is continued for some distance 



c 



-/ 



p /^^^ 



FIG. 9i. Diagram of development of the lens ABC. 

 Different stages of development. 1. Epidermic layer. 2. 

 Thickening of this layer. 3. Crystalline depression. 4. 

 Primitive ocular vesicle, its anterior part pushed back bv 

 the crystalline depression. 5. Posterior part of the primi- 

 tive ocular vesicle, forming the external layer of the sec- 

 ondary ocular vesicle. 6. Point of separation between the 

 lens and the epidermic layer. 7. Cavity of the secondary 

 ocular vesicle, occupied by the vitreous. 



FIG. 95. Diagrammatic sketch of a vertical 

 longitudinal section through the eyeball of a 

 human foetus 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 

 cornea. 1. The lens. op. The pedicle of the 

 primary optic vesicle, rp. Primary medullary 

 cavity of the optic vesicle, p. The pigment-layer 

 of the outer wall. r. The inner wall forming the 

 retina. r#. Secondary optic vesicle, containing 

 the rudiment of the vitreous humor. 



into the stalk of the optic vesicle, and thus allows a process of the mesoblast to 

 extend down the stalk to form the arteria centralis retinae and its accompanying 

 The lens is at first a thickening of the epiblast ; then a depression or involu- 



vem. 



tion takes place, thus forming an open follicle, the margins of which gradually 

 approach each other and coalesce, forming a cavity enclosed by epiblastic cells 



1 This layer forms functionally part of the choroid, and was formerly described as belonging to 

 this membrane ; it is now described as part of the retina, on account of its method of development. 



