II 



THE SKIN AND ITS DEEIVATIVES 



11 



The jaw, composed of a closely set row of such columns, is sup- 

 ported by the neighbouring parts of the epidermis which also under- 

 go a certain amount of cornification. Thus just internal to the jaw 

 is a cushion-like mass of large slightly cornified cells which forms an 

 efficient backing to it (of. Fig. 45, A and B) while external to the 

 jaw the surface, of the 



epidermis is composed ^ s> 



of flattened much 

 cornified cells (Fig. 

 45, A). 



The oral combs 

 consist of a pallisade- 

 like arrangement of 

 similar denticles which 

 however in this case 

 are not in contact. 

 Fig. 45, C, shows a 

 longitudinal section 

 through the posterior 

 labial comb of Palu- 

 dicola. Here again 

 we see a succession- 

 column of epidermal 

 cells commencing with 

 a small initial cell 

 near the inner sur- 

 face of the epidermis. 

 From the initial cell 

 outwards the cells 

 increase in size, become 

 gradually cornified and 

 each one fits closely 

 into the base of the 

 next one which be- 

 comes more and more 

 deeply excavated as 

 the tip is approached. 



Two conspicuous 

 differences distinguish 

 the denticle of the 

 oral comb from that of 

 the jaw: (1) instead 

 of being regularly conical in shape it is claw-shaped with serrated 

 edges (Fig. 45, D) the tip being recurved, and (2) the hollow base 

 of the cornified cell is not entirely occupied by its successor in the 

 series: it also accommodates an indifferent cell of the epidermis 

 (supporting cell of Gutzeit) which bulges into it. 



I have described the development of these interesting structures 



Fig. 46. — Vertical section through lingual spine of 

 Petromyzon. (After Warren, 1902.) 



A, earlier, B, later stage, d, dermis ; m.p, mesodermal 

 papilla ; s, functional spine ; si, spine rudiment. 



