318 ON SENSATION AND THE UNITY OP 



of the nose and that of the ear, the young embryo 

 presents a depression of the general integument; 

 but, in man and the higher animals, this does not 

 give rise to the proper sensory organ, but only to 

 part of the accessory structures concerned in 

 vision. In fact, the depression, deepening and be- 

 coming converted into a shut sac, produces only 

 the cornea, the aqueous humour, and the crystal- 

 line lens of the perfect eye. 



The retina is added to this by the outgrowth of 

 the wall of a portion of the brain into a sort of 

 bag, or sac, with a narrow neck, the convex bottom 

 of which is turned outwards, or towards the 

 crystalline lens. As the development of the eye 

 proceeds, the convex bottom of the bag becomes 

 pushed in, so that it gradually obliterates the 

 cavity of the sac, the previously convex wall of 

 which becomes deeply concave. The sac of the 

 brain is now like a double nightcap ready for the 

 head, but the place which the head would occupy 

 is taken by the vitreous humour, while the layer 

 of nightcap next it becomes the retina. The cells 

 of this layer which lie farthest from the vitreous 

 humour, or, in other words, bound the original 

 cavity of the sac, are metamorphosed into the rods 

 and cones. Suppose now that the sac of the 

 brain could be brought back to its original form; 

 then the rods and cones would form part of the 

 lining of a side pouch of the brain. But one of 



