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 



corneal region 



optic stalk 



pigment 

 layer of retina 



sensory 

 layer of retina 



devtloping 

 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 



