THE EMBRYOLOGY OF THE EYE. II 



of the anterior primary vesicle — which subse- 

 quently becomes the second secondary cere- 

 bral vesicle — bulges out to form the primary 

 optic vesicle. From the primary optic vesicle 

 develop the pigment-epithelium and the retina 

 proper, which latter may be divided into, ist, 

 the cerebral layer (nerve-fibres, ganglion-cells, 

 etc.), conducting elements, and,' 2d, the layer 

 of modified neural or sensory epithelium (outer 

 nuclear layer, rods and cones), which makes 

 up the percipient elements of the organ of 

 vision. The lens and conjunctiva originate 

 in the external epiblast, and the remaining 

 structures of the eye arise from the mesoblast. 

 Soon after the formation of the optic vesi- 

 cle, the auditory vesicle appears near the fifth 

 secondary cerebral vesicle. The auditory vesi- 

 cle, however, is not formed by a bulging of 

 the neural epiblast composing the cerebral 

 vesicle wall, but by an indipping of the ex- 

 ternal epiblast, forming a closed sac and after- 

 wards becoming separated from the external 

 epiblast. This sac becomes the epithelial 



