THE SKELETON 67 



SKELETON OF THE LIMBS 



So far as their skeleton is concerned, the fore and hind limbs 

 of Men and other Vertebrates, notwithstanding their various 

 adaptive modifications, are unmistakably built on the same plan. 

 This fact not only finds its expression in the strictly homologous 

 segmentation of their free portions, but is confirmed by Compara- 

 tive Anatomy and Ontogeny. 



Without entering at length into the old controversy as to 

 the phylogeny of the limbs, I would briefly define my own posi- 

 my. 



FIG. 47. TRANSVERSE SECTION THROUGH THE EMBRYO OF A SHARK (Pristiurus 



mdanostomus), 9 mm. long, showing the mode of origin of the Pectoral Limb Bud (ap. ) 



ch., notochord ; co., ccelom ; ?., myomeres, seen to be growing 



ventrally ; my. , spinal cord. 



tion with regard to this question. I agree with Balfour and 

 Dohrn in regarding the limbs of the Vertebrates as outgrowths 

 of the primitive body segments, and thus believe in their originally 

 segmental nature ; and I see in this an argument for the origin 

 of existing Vertebrates from segmented Invertebrate ancestors. 

 In other words, these limbs, which in origin are polymerous, 

 involve phylogenetically a certain number of body segments with 

 their muscles and nerves ; and these, in consequence of functional 

 adaptation, must necessarily undergo different modifications in 

 the different groups of Vertebrates. Although this subject 

 cannot be further discussed here, it may be remarked, in passing, 

 that the differences between the anterior and posterior limbs, 

 resulting from adaptive modification, become less marked the 

 lower we descend in the vertebrate series ; indeed, a starting-point 



