166 THE DEVELOPMENT OF THE CHICK 



is established the secondary optic vesicle or optic cup {aipula 

 optica). Special attention must be given to the form of the in- 

 vagination, for this determines relations of fundamental impor- 

 tance. The invagination may be stated to consist of two parts. 

 The first is directly internal to the lens primordium, and the second, 

 which is continuous with the first, involves the ventral wall of 

 the primary optic vesicle as far as the optic stalk. Two parts 

 may thus be distinguished in the mouth of the optic cup — (1) an 

 external part, which becomes the pupil of the eye, and (2) a ventral 

 part, continuous with the pupil, which is known as the choroid 

 fissure. Figs. 96 A, B, and C exhibit these relations better than 

 a detailed description. 



The choroid fissure is a transitory embryonic structure, sub- 

 sequently closing by fusion of its lips. However, it establishes 

 a relation of fundamental importance in that the ventral wall 

 of the optic-stalk is kept continuous in this way with the inner 

 or retinal layer of the secondary optic vesicle (Figs. 96 B,and 97), 

 and thus a path is provided for the development of the optic 

 nerve (see Chap. IX). It also provides an aperture in the wall 

 of the optic cup for the entrance of the arteria centralis retinae. 



The optic primordium at the 36 s stage, with the omission of 

 the lens, is composed as follows: 



(1) Optic-stalk attached to the floor of the brain; this is 

 still tubular. 



(2) The optic cup or secondary optic vesicle consisting of 

 two layers, viz., (a) a thick internal or retinal layer continuous 

 at the pupil and choroid fissure with (6) the thin external layer. 

 The cavity of the cup is the future posterior chamber of the eye; 

 it has two openings, viz., the pupil filled by the primordium of 

 the lens, and the slit-like choroid fissure extending from the pupil 

 to the optic stalk along the ventral surface. The retinal layer 

 is continuous with the floor of the optic-stalk, and thus with the 

 diencephalon. 



The optic cup expands with extreme rapidity between the 

 stages of 26 and 36 somites, as may be seen from the figures by 

 comparing the relative size of the lens and optic cup at different 



stages. 



The Lens. The invagination of the thickened ectoderm 

 external to the optic vesicle soon leads to the formation of a deep, 

 thick-walled pit which rapidly closes (26-28 somites) and thus 



