STRUCTURE OF FOUR-DAY CHICKS 



121 



of the cup is incomplete, the gap in it being known as the choroid 

 fissure (Figs. 35 and 36, B). 



The lens arises as a thickening of the superficial ectoderm 

 which becomes depressed to form a vesicular invagination ex- 

 tending into the optic cup (Fig. 36, B). 



ectoderm 



diocoele 

 mesenchyme 



concentration of 



mesenchyme 

 pigment layer 

 sensory layer 



lens 



area enlarged in B 



corneal region 

 optic stalk 



" 



P-gr 



* 





pigment 

 layer of retina 



sensory 

 layer of retina 



developing 

 lens fibers 



FIG. 45. Drawings to show structure of the eye of a four-day chick. 

 A, diagram to show topography of eye region; B, drawing to show cellular 

 organization of the pigment and sensory layers of the retina. Abbreviations: 

 mes., mesenchymal cell; p.gr., pigment granule; C, drawing to show cellular 

 organization of the lens. 



In chicks of four days the choroid fissure has become nar- 

 rowed by the growth of the walls of the optic cup on either side 

 of it (Figs. 40 and 42, B). The orifice of the optic cup becomes 



