THE OUTER TISSUES OF ANIMALS. 301 



of the ecderon, within which they are formed, and by which 

 the former are, at first, entirely enclosed." And though the 

 scales of fishes and the dermal plates of reptiles present diffi- 

 culties, yet Prof. Huxley concludes that the course of their 

 development is at first essentially the same. Some idea of it, 

 and of the relations it proves among these structures, may be 

 given thus : — Suppose a small pit to be formed on the pre- 

 viously flat skin ; and suppose that the growth and casting 

 ofi' of horny cells which goes on over the skin in general, 

 continues to go on at the usual rate over the depressed surface 

 of this pit. Clearly the quantity of horny matter produced 

 within this hollow, will be greater than that produced on a 

 level portion of the skin subtending an equal area of the 

 animal's outside. Suppose such a pit to be deepened 

 until it becomes a small sac. If the exfoliation goes on as 

 before, the result will be that the horny matter, expelled, as 

 it must be, through the mouth of the sac, which now bears 

 a small proportion to the internal surface of the sac, will be 

 large in quantity compared with that exfoliated from a 

 portion of the skin equal in area to the mouth of the sac : 

 there will be a conspicuous thrusting forth of horny matter. 

 Suppose once more that the sac, instead of remaining simple, 

 has its bottom pushed up into its interior, 'il^e the bottom of 

 a beer-bottle — the introversion being carried so far that the 

 introverted part reaches nearly to the external opening, and 

 leaves scarcely any space between the introverted part and 

 the walls of the sac. It is easy to see that the exfoliation 

 continuing from the surface of the introverted part as well as 

 from the inside of the sac generally, the horny matter cast 

 off will form a double layer ; and will come out of the sao 

 in the shape of a tube having within its lower end the intro- 

 verted part, as the core on which it is moulded, and from the 

 apex of which is cast off the substance filling, less densely, 

 its interior. The structure resulting will be what we know 

 as a hair. Manifestly by progressive enlargement of the sac, 

 nnd further complication of that introverted part on which 



