. Text-book of embryology. Embryology. ii THE SKIN AND ITS DEKIVATIVES 75 arise as special developments of the horny layer ensheathing 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) (Fig. 43, C, ), while on the ventral side it forms the softer " sole " of the claw (Boas) (Fig. 43, C, ). Neonychia or Claw-pads.—To the embryo of an Amniotic Verte- brate, enclosed within its delicate membranes, the possession
. Text-book of embryology. Embryology. ii THE SKIN AND ITS DEKIVATIVES 75 arise as special developments of the horny layer ensheathing 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) (Fig. 43, C, ), while on the ventral side it forms the softer " sole " of the claw (Boas) (Fig. 43, C, ). 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 Bathke (1866), Voeltzkow FlG- 44.—Right pectoral (.1899) and Goeldi (1900) in Crocodilian () eote aCttvo^onZ embryos are the same structures and it seems after oviposition, show- probable that they will be found to OCCUr in ing the hoof-like neony- claw-bearing Amniotic Vertebrates generally. 1899.) ^ °6 Z'°W' 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 t
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