DEVELOPMENT OF THE CHICK 131 



the nervous elements of the eyes are produced. 

 The cavity of the optic vesicle is at first widely 

 continuous with the cavity of the fore-brain, 

 but as development proceeds the connection 

 is narrowed and a hollow optic stalk is pro- 

 duced. The optic vesicle is converted into a 

 cup by an invagination of its more distal 

 part. The cavity between the two walls of 

 the cup thus produced is obliterated and 

 the hollow of the optic stalk disappears as 

 a consequence of the formation of the fibres 

 of the optic nerve. The inner layer of the 

 cup becomes the retina of the fully formed 

 eye. 



The ectoderm of the surface of the head 

 becomes invaginated and thickened opposite 

 the optic cup, thus producing a pit which 

 soon closes and forms a sac. The cavity of 

 the sac disappears, the cells composing the 

 wall of the sac become modified, and the 

 lens is formed. 



Opposite the ninth, tenth, and eleventh 

 neuromeres the surface ectoderm begins to 

 thicken about the time when twelve meso- 

 blastic somites have been formed. An auditory 

 pit is thus produced by invagination, and, by 

 the gradual closing of the mouth of the pit, 



