THE INTEGUMENT AND THE EXOSKELETON 95 



It thus appears that, aside from the sporadic distribution of 

 scales in various mammals, the palmar and plantar surfaces, 

 save in the most modified cases (Ungulates, Cetacea), gre^ 

 covered with scale elements, either distinct or united in rows 

 to form ridges; and, furthermore, that in other parts of the 

 body the hair follicles occur in definite groups, arranged in 

 alternate series; facts that can be interpreted only as indica- 

 tive of the former presence of a scaly coat. 



That this stage is actually passed through in embryo mam- 

 mals has not as yet been definitely determined, but some cir- 

 cumstances seem to indicate that the vestiges of this covering 

 may be looked for in the epitrichium, which is a superficial 

 epidermic formation without definite structure so far as is 

 known. This at one time covers the surface, but save in the 

 palmar and plantar regions disintegrates and disappears ; con- 

 tributing in man to the formation of the vernix caseosa, found 

 upon the surface of the new-born infant. Upon the palms 

 and soles, however, at least in man, where it has been mainly 

 studied, it appears to persist and take part in the formation 

 of the friction ridges. 



This brings with it the suggestion that the epitrichium rep- 

 resents the primitive scaly coat of ancestral mammals, 

 greater part of which becomes lost by an embryonal ecd\sis. 

 How this epitrichium appears and what its fate is on surfaces 

 where scales persist, other than the palms and soles, or in the 

 few scaled mammals, is not known; but in the sloths and 

 ant-eaters, nearly related to the last, it is especially firm and 

 remains until birth as a definite covering. In many mammals 

 the similarity to a moulting external layer is increased by the 

 presence of a thick layer over the nails or claws, continuous 

 with the epitrichium, and cast off with it, the eponychium. 



The hair, which forms the characteristic coat of present-day 

 mammals, may be safely considered as once accessory to a 

 covering of scales, which it has secondarily replaced, as ex- 

 plained in the foregoing, but this does not account for its 

 origin, or suggest its primary function. An attempt has been 

 made to homologize hairs with the integumental sense organs 



