ii THE SKIN AND ITS DEEIVATIVES 75 



arise as >).<( ial developments of the horny layer ensheathin^ the tip 

 of the digit. To produce and retain a sharp edge or point by differ- 

 ential wear the claw is normally of denser consistency and harder on 

 the dorsal side and laterally, forming the " claw-plate " (Boas) (Fr_ 

 C, c.p), while on the ventral side it forms the softer " sole " of the 

 claw (Boas) (Fig. 43, C, c.s). 



Neonychia or Claw-pads. To the embryo of an Amniotic Verte- 

 brate, enclosed within its delicate membranes, the possession of sharp 

 claws on the digits would obviously be a source of considerable 

 danger during the later stages of development when the embryo 

 moves its limbs, because of the liability of such structures to tear 

 the foetal membranes. This danger is obviated by a beautiful 

 adaptive arrangement which has been described 

 by Agar (1909). 



In the embryo, the concavity on the lower 

 side of the claw is completely filled up by a 

 soft rounded pad or cushion (Fig. 43, A, n) 

 formed by a thickening of the horny layer of 

 the epidermis superficial to the sole of the claw. 

 Agar has given the name Neonychium to this 

 structure. In addition to mammals, which do 

 not concern us here, Agar has studied these 

 claw-pads in the Fowl and in the Lizard Tejus 

 and there can be no doubt that the expanded 

 claw-tips observed by Kathke (1866), Voeltzkow Fl( ;: 44. Right pectoral 



/-inrvrvx jn i j /-i r\ A^N n TT /T-- IA\ limb of an embryo Cro- 



(1899) and Goeldi (1900) in Crocodilian (Fig. 44) codile about two months 

 embryos are the same structures and it seems after oviposition, show- 

 probable that they will be found to occur in ing the hoof-like neouy- 



i , J . . . -v-r . , ,, ehia. (After Voeltzkow, 



claw-bearing Amniotic Vertebrates generally. 1399.) 



The neonychia are purely foetal structures 



which become detached soon after hatching (Fig. 43, B and C) leaving 

 behind the functional claw. 



Jaws and Oral Combs of Anuran larvae. Amongst the most 

 interesting developments of the horny layer are the jaws and oral 

 combs of frog tadpoles. The buccal opening is bounded by an upper 

 and lower horny jaw, and external to and roughly parallel with 

 these are rows of little horny denticles which form the oral combs 

 and are used for fraying out the food. The number and arrange- 

 ment of these rows of denticles "upper labial" and "lower labial" 

 differs in different Anura and they afford useful characters for the 

 identification of tadpoles (see Boulenger, 1897). 



The horny jaw is composed simply of a row of denticles so closely 

 apposed as to be in contact. The terminal functional portion of each 

 denticle is seen in longitudinal section (Fig. 45, A and B) to be com- 

 posed of a series of hollow cones of hard horny material which closely 

 ensheath one another. The terminal cone as it undergoes wear and 

 tear eventually drops off, its function being taken over by the cone 

 which it previously ensheathed. 



