264 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



nor in serpents, nor in tailed amphibians, nor in the em- 

 bryos of mammals, nor even in the embryo of man. So 

 that their absence is only an extreme term of modifica- , 

 tion, all the steps of which may be found. Going still 

 back, the sacrum may be regarded as five to six vertebrae 

 with their bodies — neural arches and visceral arches — all 

 consolidated. Indeed they are distinct, and have their 

 ribs in many lower vertebrates and in the embryos of 

 mammals and man.* 



Signification of Limbs. — It will be observed that 

 in this scheme we have left out the limbs. This seems 

 a very great omission, but this is so only in the higher 

 vertebrates. The primal vertebrates were probably limb- 

 less. And in all the earliest vertebrates and in the 

 lowest vertebrates to-day limbs are comparatively in- 

 significant appendages, which may be left out in any 

 general scheme of skeletal structure. It is probable 

 that limbs can not be brought into the original plan of 

 homologous segments, but have been added afterward 

 as a sort of afterthought. 



Origin of Limbs. — Professor Owen f thought to 

 bring limbs into this scheme of repeated segments by 

 making them appendages to the visceral arches, and 

 this view is expressed in his figure of the archetypal ver- 

 tebrate (Fig. 167), where small appendages are seen on 

 every arch. He believed that the shoulder girdle and 

 hip girdle were formed by consolidation of several vis- 

 ceral arches, and that the limbs were greatly enlarged 

 appendages to these arches, the appendages to the 

 other arches being still rudimentary or wanting. But 

 this view, though very suggestive, is not now generally 



* Cervical, lumbar, and sacral ribs are found in the embryo of 

 mammals and of man. Wiedersheim, p. 51. 



t Owen, " Homologies of Vertebrate Skeletons " and " Signifi- 

 cation of Limbs." 



