616 THE WONDER OF LIFE 



rehabilitation of the oldest mode of tooth- development. 



Amphibian Scales. Sometimes it seems quite legiti- 

 mate to recognize a structure as a relic although we are 

 not aware of the precise affiliation. Every one knows 

 that almost all amphibians are naked- skinned or scaleless 

 in great contrast to the scaly Reptiles. It is also well 

 known that some of the ancient extinct Amphibians, the 

 Labyrinthodonts or Stegocephali, were armoured. There- 

 fore it is interesting to find in the most old-fashioned stocks 

 of living Amphibians, namely the burrowing Csecilians, 

 that there are transverse rows of thin calcified scales 

 imbedded in the dermis or under-skin. Moreover, in a few 

 rare cases among tailless Amphibians, there are bony scales 

 in the skin. Thus in CeratopJirys dorsata there is a bony 

 shield on the back which arises from the confluence of a 

 large number of small ossifications in the dermis. It is 

 very unlikely that this can mean anything but a retention 

 of the ancestral armour. Not less interesting, though less 

 secure, is Margarethe Kressmann's interpretation of 

 numerous papillae that occur all over the lower layer of 

 the dermis in Siren lacertina, the American mud- eel. 

 Each consists of firm connective tissue and is usually tipped 

 with a mantle of pigment. They project into the more 

 superficial looser layer of the dermis and are quite hidden 

 from the outside. It seems reasonable to interpret them 

 as dwindling vestiges of the ancestral armature. 



The Egg Tooth. At the tip of the bill of many un- 

 hatched young birds there is a horny knob which is called 

 the egg- tooth. It has nothing whatever to do with teeth, 

 of which, as separate structures, no living bird is known 

 to show any hint (the alleged cases of tern, etc., having 

 broken down) ; but it is interesting in several ways. If it 



