Development of the Third Day 183 



becomes folded to form the fan-like structure 

 of the adult pecten. Before the time of hatch- 

 ing it becomes deeply pigmented. The pec- 

 ten is a structure characteristic of the eyes of 

 birds and of many reptiles : its exact function 

 is not known with certainty (Fig. 61). 



The cornea, which is apparently simply a 

 continuation of the sclerotic coat, is made up 

 of mesoblast which grows in between the lens 

 and the superficial epithelium. It is, at first, 

 structureless, but certain of the mesoblast 

 cells become converted into the corneal cor- 

 puscles. These corpuscles form a layer in the 

 middle part of the thickness of the cornea, 

 while the inner and outer surfaces remain 

 structureless and form the anterior and poste- 

 rior membranes. The layer of surface epithe- 

 lium persists as the conjunctival epithelium. 



The anterior chamber of the eye forms 

 between the lens and the cornea, and in 

 this chamber the aqueous humor collects. 



" The eyelids are folds of integument around the 

 eye; there are three of them, an upper and a lower 

 eyelid, and the third eyelid or nictitating membrane 

 which arises on the inner or nasal side of the eye. 

 The lacrymal glands are solid ingrowths of the con- 

 junctival epithelium, which appear on the eighth 



