502 HISTORY OF THE HUMAN BODY 



The functional lens is formed by the cornification of these cells, 

 and the mass thus formed is covered anteriorly with a thin 

 epithelium, the original anterior wall of the vesicle. 



It will be noticed that there is in this development of the 

 lens a striking similarity with the early stages of both the nose 

 and the ear, and if there be taken in connection with these cer- 

 tain temporarily thickened areas of the external ectoderm in 

 association with the Facialis and the Glosso-pharyngeus 

 nerves, which appear and vanish again during the embryonic 

 life of the lower vertebrates, the idea comes at once to mind 

 that we have here the record of a series of ancient sense-organs 

 laterally placed, perhaps a pair for each metamere, some of 

 which have specialized in various ways while others have be- 

 come lost. If this be true, the lens was originally, not a re- 

 fracting medium, but a sense-organ itself, which has given up 

 its primary function entirely and entered the service of another 

 sense-organ, different in origin from that of any other in ver- 

 tebrate history, namely, a specialization of a portion of brain 

 surface. 



The idea of this ancient series of sense-organs suggests 

 many questions. What was the primary function of this 

 series? Did these sense-organs sustain any relation to trie 

 lateral line organs ? To these questions, belonging themselves 

 to the realm of pure suggestion, we can give but speculative 

 answers. Both the nose and the ear, as we have already seen, 

 have in their structure and development something to suggest 

 a kinship with the lateral line organs ; this is especially true of 

 the latter, with its nerve appearing in connection with that ele- 

 ment of the facial nerve that supplies these organs in fishes, 

 and with its semicircular canals that resemble the canals of 

 Lorenzini. The eye itself cannot, of course, be included in any 

 series of sense-organs of integumental origin, but the lens can, 

 and there seems no intrinsic difference, up to a certain stage, 

 between the lens capsule and that of the inner ear. The nasal 

 sacs, again, are similar capsules that do not lose their connec- 

 tion with the exterior, and it must be remembered that in the 

 endolymphatic duct of the selachians we see the same retention 



