1879.] on the Unity of Structure of Sensiferons Onjaiis. 123 



part of the head, lined by a continuation of the general epidermis. 

 These depressions become pits, and the pits, by the growth of the 

 adjacent parts, gradually acquire the position which they finally 

 occupy. The olfactory organ, therefore, is a specially modified part 

 of the general integument. 



The human ear would seem to present greater difficulties. For 

 the essential part of the sense organ, in this case, is the membranous 

 labyrinth, a bag of complicated form, which lies buried in the depths 

 of the floor of the skull, and is surrounded by dense and solid bone. 

 Here, however, recourse to the study of development readily unravels 

 the mystery. Shortly after the time when the olfactory organ appears 

 as a depression of the skin on the side of the fore part of the head, 

 the auditory organ appears as a similar depression on the side of its 

 back part. The depression, rapidly deepening, becomes a small pouch, 

 and then, the communication with the exterior becoming shut off, the 

 pouch is converted into a closed bag, the epithelial lining of which is 

 a part of the general epidermis segregated from the rest. The adjacent 

 tissues, changing first into cartilage and then into bone, enclose the 

 auditory sac in a strong case, in which it undergoes its further meta- 

 morphoses ; while the drum, the ear bones, and the external ear are 

 superadded by no less extraordinary modifications of the adjacent 

 parts. Still more marvellous is the history of the development of the 

 organ of vision. In the place of the eye, as in that of the nose and 

 that of the ear, the yoimg 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, this depression, 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 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 furthest 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 eye are modified 



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