280 THE CHICK. 



fan-like manner characteristic of the adult ; and toward the close 

 of incubation it becomes densely pigmented. 



The cornea is formed from mesoblast, which grows in between 

 the lens and the surface epiblast, at first as a ring, but soon 

 becoming a continuous layer across the front of the eye. It is 

 at first structureless, but cells from the mesoblast round its 

 edge soon grow inwards into its substance to form the corneal 

 corpuscles. These corpuscles are confined to the middle layer 

 of the thickness of the cornea, the outer and inner surfaces 

 remaining structureless as the anterior and posterior elastic 

 membranes of the cornea respectively. The surface layer of 

 epiblast persists as the conjunctival epithelium. 



The anterior chamber of the eye forms as a space between 

 the cornea and the lens ; and in it a watery fluid, the aqueous 

 humour, soon collects. 



The accessory organs of the eye. The eyelids are folds of the 

 integument round the eye : there are three of them, an upper 

 and a lower eyelid, and the third eyelid or nictitating membrane 

 (Fig. 126, 'CD), which arises on the inner or nasal side of the 

 eye. The lacrymal glands are solid ingrowths of the conjunctival 

 epithelium, which appear on the eighth day. The lacrymal duct 

 is also at first solid ; it appears as a ridge of epidermis, along 

 the line of the lacrymal groove, extending from the eye to the 

 olfactory pit (Fig. 125). This ridge sinks into the mesoblast, 

 and soon splits off from the epiblast along the greater part of its 

 length, but remains attached at its ends to the lower eyelid and 

 to the wall of the olfactory pit respectively. About the twelfth 

 day it acquires a central lumen, and becomes the tubular duct. 



3. The Ear. 



The ears appear, about the middle of the second day, as a pair 

 of shallow depressions of the external epiblast at the sides of the 

 hind-brain, just in front of the first pair of mesoblastic somites 

 (Figs. Ill and 120, EI). The pits rapidly deepen (Fig. 121, EI) ; 

 their mouths narrow, and by the end of the third day become com- 

 pletely closed, the pits thus becoming vesicles imbedded in the 

 mesoblast at the sides of the head (Fig. 113. EI). By a series of 

 changes very similar to those already described in the frog, the 

 vesicle gives rise to the various parts of the membranous laby- 

 rinth of the ear ; the epiblastic wall forming the epithelial lining of 



