THE OUTER TISSUES OF ANIMALS. 305 



lining that part of the chamber is developed from it in the 

 same way that the substance of a hair is developed from the 

 papilla at the bottom of its sac. The iris originates as an 

 annular thrusting-in of the walls of this chamber in front of 

 the crystalline lens ; and between the two layers of the epi- 

 dermic lining, thus folded, comes a portion of the derma in 

 which muscular fibres eventually arise. Though the founda- 

 tion of the part behind the crystalline lens is laid by a hollow 

 diverticulum from the brain, which grows outwards to meet the 

 inward-growing tegumentary sac, yet here, too, structures be- 

 longing to the tegumentary system eventually predominate. 

 For into this cul-de-sac proceeding from the nervous centre, 

 there takes place a lateral growth of dermal tissue, which, in- 

 troverting the wall of the sac, and presently filling the whole 

 cavity of it, is at last shut off by the closure of the now 

 doubled walls of the sac ; and out of this intruding mass of 

 dermal tissue the vitreous humour is formed. That is to say, 

 the eye considered as an optical apparatus is wholly produced 

 by metamorphoses of the skin : the only parts of it not thus 

 produced, being the membranes lying between the sclerotic 

 and the vitreous humour, including those retinal structures' 

 formed in them. All is tegumentary save that which has to 

 appreciate the impressions which the modified integument 

 concentrates upon it. 



Thus, as Prof. Huxley has somewhere pointed out, there 

 is a substantial parallelism between all the sensory organs in 

 their modes of development : as there is, too, between their 

 modes of action. A vibrissa may be taken as their common 

 type. Increased impressibility by an external stimulus, 

 requires an increased peripheral expansion of the nervous 

 system on which the stimulus may fall ; and this is secured 

 by an introvertion of the integument, forming a sac on the 

 walls of which a nerve may ramify. That the more extended 

 sensory art a thus constituted may be acted upon, there 

 requires some apparatus conveying to it from without the 

 appropriate stimulus ; and in the case of the vibrissa, this 



