100 THE MAMMOTH CAVE. 



nized mode of its development (since it is 

 usually formed from an involution qf the skin), 

 unless we suppose that after the folding of the 

 skin had taken place in the embryonic con- 

 dition the lens retreated from the surface and 

 all. connection with the integument ceased. 



•"According to Quatrefages, however, the eye 

 of Amblyopsis is contained wholly in the cavity 

 of the dura mater, and yet it has all the appear- 

 ance of being provided with a lens. If his 

 description be correct, then the mode of devel- 

 opment as well as the morphology of the eye 

 in this remarkable fish is different from that 

 of most other vertebrates, since the lens never 

 could have been formed from an involution of 

 the skin, nor could the eye, with its lens, as 

 Professor Owen asserts, be a modified cutaneous 

 follicle. That there should be different modes 

 of development of parts of the eye in different 

 animals is by no means improbable, since we 

 find this actually to be the case in another 

 organ of sense, the nose. In some fishes the 

 nostrils result from a depression or involution 

 of the skin simply, and do not at any period 

 communicate with the mouth ; while in all the 

 higher vertebrates they are formed by sub- 

 division of the primitive oral cavity. It is 



