X.] SENSATION AND THE SENSIFEKOUS OKGANS. 271 



that of the ear, the young embryo presents a depres- 

 sion 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, this depres- 

 sion, deepening and becoming converted into a shut 

 sac, produces only the cornea, the aqueous humour, 

 and the crystalline 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 con- 

 vex bottom of the bag becomes pushed in, so that it 

 gradually obliterates the cavity of the sac, the pre- 

 viously 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 the most wonderful revelations of 

 embryology is the proof of the fact that the brain 

 itself is, at its first beginning, merely an infolding of 

 the epidermic layer of the general integument. Hence 

 it follows that the rods and cones of the vertebrate 



