THE SKELETAL SYSTEM 



187 



ferences are those of pro{)ortion only. The facts are in harmony with the 

 assumption that the two have evolved from a common ancestry. 



Homologies of the Limb Bones. The striking similarity of the bones 

 of the upper and lower limbs, notwithstanding their great diversity of 

 function, is interpreted by morphologists as indicating a primary similarity 

 in use. Their present differences in form, size, and function have arisen 

 secondarily and adaptively. 



Development of the Appendicular Skeleton. The paired appendages 

 of vertebrates arise from two Wolffian folds, which extend along the 

 sides of the embryo at approximately the level where the hypomere 



NCURAL PROC£33. 



1 1 MM. EMBRYO 



Fig. 177. — Stages in the development of the appendicular skeleton of man. A, left 

 lateral aspect of arm in 11 mm. embryo; B, left lateral aspect of arm in 16 mm. embryo; 

 C, left lateral aspect of arm in 20 mm. embryo; D, left lateral aspect of leg in 11 mm. 

 embryo; E, left lateral aspect of leg in 14 mm. embryo; F, left lateral aspect of leg in 

 20 mm. embryo. (Redrawn after Bardeen and W. H. Lewis from Keibel and Mall.) 



connects wath the mesomere. Only the end portions of these folds, how- 

 ever, go to form the Umbs; the intermediate region atrophies and disap- 

 pears. The e\ddence accords with expectation from the standpoint of the 

 fin-fold theory of the origin of the extremities. See p. 177. 



The Wolffian folds consist of an external covering of ectoderm and a 

 core of mesenchyma, which in the human embryo is of uncertain origin. 

 In their early development, both arms and legs take the form of shovel- 

 shaped outgrowths (Fig. 176), which gradually elongate. The cartilagi- 

 nous anlagen of the bones arise in the mesenchyma (Fig. 176.4) and are 

 slowly converted into bone through complex processes partly suggested in 



