BVOLUnON OF THE SKULL. 299 



hyoidea) and its great horn on each side. The fourth gill- 

 arch appears in the mammalian embryo only as a transient, 

 rudimentary embryonic organ, and does not develop into 

 special parts. Of the posterior gill-arches (the fifth and 

 sixth pairs), which are permanent in the Primitive Fishes, no 

 trace is visible in the embryo of higher Vertebrates. The 

 latter have long been lost. The four gill-openings in the 

 human embiyo are also only interesting as transient rudi- 

 mentary organs, which soon disappear entirely by concre- 

 scence. The first gill-opening (between the first and second 

 gill-arches) alone is of permanent importance ; from it 

 develops the drum, or tympanic cavity of the ear, and the 

 Eustachian tube. (Cf. p. 269, and Plate I., with explan- 

 ation.) 



Not only did Gegenbaur, in his model " Researches into 

 the Comparative Anatomy of Vertebrates," fii'st correctly 

 explain the skull and its relation to the vertebral column, 

 but he also first performed the no less weighty and interest- 

 ing task of showing the phylogenetic derivation of the 

 skeleton of the Hmbs in aU Vertebrates from one primordial 

 form. Few parts of the body in the difierent Vertebrates 

 are subjected, by adaptation to various circumstances, to 

 such an infinite variety of modifications as the limbs, in 

 point of size, form, and speciaL fitness for certain purposes, and 

 yet we are now able to refer them all to one common here- 

 ditary form. Vertebrates are distinguishable as regards the 

 structure of their limbs into three large main groups. The 

 lowest and most ancient Vertebrates, the skull-less and jaw- 

 less classes, like all their invertebrate ancestors, had no 

 paired limbs ; this condition is yet represented in the Am- 

 phioxus and in the Cyclostomi (Figs. 189, 190). The second 



