268 JAMES ROLLIN SLONAKER 



As development advances this fissure gradually narrows by 

 the proliferation of the cells of the walls in this region, so that 

 by the time the chick has reached the age of ninety-six hours it 

 is a mere cleft (text-fig. 5, Chs). The edges of the walls of the 

 fissure have not only grown closer together, but the lower rim of 

 the optic cup, which was retarded, has grown forward until the 

 whole front of the rim is symmetrical. The walls of the chorioid 



Chs. 



Text fig. 5 Side view of chick of about ninety-six hours' incubation. CB, 

 cerebellum; CH, cerebral hemispheres; Chs, chorioid fissure; E, ear pit; FB, 

 fore-brain; L, lens; MB, mid-brain; Opv, optic vesicle; s-s, plane of section of 

 figure 3. 



fissure unite first at the rim of the optic cup, the union continuing 

 inward until the fissure is completely closed. 



In the earliest embryo sparrow secured (figs. 1 and 16) the eye 

 is shghtly further developed than in the 100-hour chick, although 

 the chorioid fissure (c) still is visible. Figure 48 is a photograph 

 of a section through the eye at this age. The optic stalk (OpSt) 

 is plainly visible. The original optic vesicle has completely 

 closed by the apposition of the anterior and posterior walls. The 

 cavity of the lens has been obliterated by the rapid growth of 



