29S THE EVOLUTION OF MAN. 



leather-skin (corium), and only secondarily, come into closer 

 relations with tlie skull. How, in Man, this most simple and 

 primordial rudiment of the primitive skull develops, onto- 

 genetically, from the head-plates, and how, in the mean- 

 time, the anterior extremity of the notochord is enclosed in 

 the base of the skull, has already been explained. (Cf 

 vol i. p. 378; Figs. 145 and 146, p. 393.) 



The main features in the history of the development of 

 tlie gill-arches, which must now be regarded as skull-ribs, 

 lias been told. Of the four original rudimentary gill-arches or 

 Mammals (Plates I. and VII., Figs. 232-236, p. 243), the first 

 lies between the primitive mouth-opening and the first gili- 

 opening. From the base of this gill-arch the " upper jaw 

 process " develops, and this unites, in the manner already 

 described, with the internal and the external nasal processes 

 on each side, and forms the chief parts of the upper jaw skele- 

 ton palate-bones, wing-bones, etc. (Cf p. 245 and 208.) The 

 rest of the first gill-aich, now distinguished as the " lower-jaw 

 process," forms out of its base two ear bonelets — the hammer 

 (malleus) and the anvil (incus) ; the rest of its mass becomes 

 a long strip of cartilage, called, after its discoverer, " Meckel's 

 cartilage." On the external surface of this cartilage origin- 

 ates, as a surface-bone (formed of cellular matter from the 

 leather-plate), the permanent bony lower jaw. From the 

 base of the second gill-arch in Mammalia originate the 

 third eai" bonelet, the stirrup (stapes), and from the subse • 

 quent parts, in order, the stirrup-muscle, the styloid process of 

 the temporal bone, the styloid band, and the small horn of the 

 tongue-bone. Finally, the third gill-arch becomes cartilagin- 

 ous only at its anterior portion, and here, by the union of iUi 

 two halves, is formed the body of the tongue-bone {cojmla 



