1/2 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



FIG. 185. Skeleton of pectoral fin of Ceratodus, after Gunther. 



Before beginning the account of the skeleton of the limbs 

 it may be well to summarize the two prominent theories of the 

 origin of these parts. According to the view of 

 Gegenbaur (which he has lately repeated), limbs 

 have arisen from gill structures which have mi- 

 grated backwards. The gill arches have given 

 rise to the girdles, while the skeletal parts of the 

 appendages have had their origin from the gill 

 rays. With the outgrowth of the limb one of the 

 gill rays near the middle of the arch has cor- 

 respondingly elongated, and in its outgrowth 

 has carried the neighboring gill rays along with 

 it, the result being a skeletal axis to the limb, 

 on either side of which was a series of smaller 

 skeletal pieces (Fig. 184). A fin closely 

 corresponding to the requisites of this view 

 is found in the dipnoan Ceratodus. By sup- 

 pression of almost all of these accessory 

 skeletal parts on one side of the axis, and 

 a modification or suppression of some 

 on the other side of this archipterygium, 

 Gegenbaur derives all types of vertebrate 

 limbs. Fig. 186 shows the relations of 

 the typical pentadactyle leg. 



The other view, first advanced by 

 Thacher, assumes that the ancestral verte- 

 brate was provided with two longitudinal 

 folds on either side of the body. The 

 more dorsal of these migrated upwards, 

 those of the two sides uniting to form the dorsal part of the 



FlG. 1 86. Diagram 

 of amphibian fore limb, 

 after Gegenbaur. c, 

 centrale ; //, humerus; 

 ft, radius ; i, interme- 

 dium ; ;-, radiale ; U, 

 ulna; , ulnare ; 1-5, 

 carpales. The heavy 

 line is the axis of Gegen- 

 baur's archipterygium, 

 the dotted lines, of the 

 radials of his scheme. 



