122 EARLY EMBRYOLOGY OP THE CHICK 



narrowed by convergence of its margins toward the lens (Fig. 

 45, A). Meanwhile the lens has become freed from the super- 

 ficial ectoderm and forms a completely closed vesicle. Sections 

 of the lens at this stage show that the cells constituting that 

 part of its wall which lies toward the center of the optic cup 

 are becoming elongated to form the lens fibers (Fig. 45, C). 



At this stage we can identify the beginning of most of the 

 structures of the adult eye. The thickened internal layer of 

 the optic cup will give rise to the sensory layer of the retina 

 (Fig. 45, B). Fibers arise from nerve cells in the retina and 

 grow along the groove in the ventral surface of the optic stalk 

 toward the brain to form the optic nerve. The external layer 

 of the optic cup gives rise to the pigment layer of the retina. 

 Mesenchyme cells can be seen aggregating about the outside of 

 the optic cup. From these the sclera and choroid coat are 

 derived. Some of the mesenchyme makes its way into the 

 optic cup through the choroid fissure and gives rise to the cellu- 

 lar elements of the vitreous body. The complex ciliary appar- 

 atus of the adult eye is derived from the margins of the optic 

 cup adjacent to the lens. The corneal and conjunctival epi- 

 thelium arise from the superficial ectoderm overlying the eye. 

 Mesenchyine cells which make their way between the lens and 

 the corneal epithelium give rise to the substantia propria of the 

 cornea. 



The Ear. — Of the structures taking part in the formation of 

 the ear, the first to appear is the auditory placode. The audi- 

 tory placode is recognizable in 36-hour chicks as a thickened 

 plate of ectoderm. Almost as soon as it appears the placode 

 sinks below the level of the surrounding ectoderm to form the 

 floor of the auditory pit (Fig. 22). By constriction of its open- 

 ing to the surface the epithelium of the auditory pit becomes 

 separated from the ectoderm of the head and comes to lie close 

 to the lateral wall of the myelencephalon (Fig. s^^ ^)' A tubu- 

 lar stalk, the endolymphatic duct, remains for a time adherent 

 to the superficial ectoderm, marking the location of the original 

 invagination (Fig. 40). 



The degree of development reached by the ear primordium 

 in four-day chicks gives little indication of the nature of the 

 later processes by which the ear is formed. The auditory 

 vesicle by a very complex series of changes will give rise to the 



