100 THE MAMMOTH CAVE. 
nized mode of its development (since it is 
usually formed from an involution of 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 
