166 THE DEVELOPMENT OF THE CHICK 
is established the secondary optic vesicle or optic cup. Special 
attention must be given to the form of the invagination, for this 
determines relations of fundamental importance. The invagina- 
tion may be stated to consist of two parts. The first is directly 
internal to the lens primordium, and the second, which is continu- 
ous with the first, involves the ventral wall of the primary optic 
vesicle as far as the optie stalk. The secondary optic vesicle 
established by these invaginations thus has two openings into 
its cavity, (1) the external opening, which becomes the pupil 
of the eve, and (2) the ventral opening, 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 optie cup for the entrance of the arteria centralis retine. 
The optic primordium at the 36s stage, with the omission of 
the lens, is composed as follows: 
(1) Optie-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 optie 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 optie vesicle soon leads to the formation of a deep, 
thick-walled pit which rapidly closes (26-28 somites) and thus 
