318 OUTLINES OF CHORDATE DEVELOPMENT 



draws together somewhat. In the middle of the cup, opposite 

 the lens, it remains open as the circular rudiment of the pupil. 

 Ventrally from this the rim draws together more closely, 

 leaving only a narrow slit-like opening extending from the 

 pupil to the attachment of the optic stalk; this is known as the 

 choroid fissure (Fig. 128). The choroid fissure remains open 

 for several days and takes an important part in determining 

 the course of the later development of the eye. 



We must now return to trace the formation of the lens. 

 The ectodermal thickening mentioned above rapidly extends, 



FIG. 128. Diagrams of sections through the eye of the chick embryo at the 

 end of the second day. After Lillie. The dorsal margin is toward the top of the 

 page in A and B. A. Eye as viewed directly. B. Vertical section through the 

 line x-cf, in A. C. Horizontal section through the line y-y, in A. cf. Choroid 

 fissure; cv, cavity of primary optic vesicle; ec, superficial ectoderm of head; i, 

 inner or retinal layer of optic cup; I, lens; o, outer or pigmented layer of optic 

 cup; ol, opening of lens sac from surface of head; pc, posterior chamber of eye; 

 s, optic stalk, continuous with the floor and lateral wall of the diencephalon. 



and proceeds to invaginate toward the invaginating retinal 

 layer. Its wall thickens as it invaginates simply, and soon 

 after the invagination of the retinal layer is completed, the 

 lens separates from the ectoderm, forming a flattened vesicle 

 lying in the cavity of the optic cup. 



These events are completed during the third day, and now 

 the optic cup enlarges very rapidly for a time, forming a large 

 cavity (posterior chamber) still open through the pupil and 

 the choroid fissure. From the rudiments now established and 

 from the surrounding mesenchyme, the parts of the fully 

 formed eye are all derived. We may mention only a few of the 



