CHAPTEE XX. 



THE LIMBS. 



The Limbs. — The limbs begin to appear at the end of the 3rd 

 week. A slight elevation or ridge is then seen to run along the 

 dorsal border of the somatopleure, at some distance from the row 



4th vise, arch 



rad. border- 

 heart 



th C. segment 



ulnar border 



£— tibial border 



5th L 



12th D. 



Pig. 233.— Lateral view of a human embryo at the 28th day, showing the 

 Limb Buds, Lateral Eidges, and Primitive Segments. 



of primitive segments formed in the paraxial mesoblast (Fig. 233). 

 The limb buds spring from this ridge as fiat processes with an 

 upper, dorsal, or extensor surface, and a lower, ventral or flexor 

 surface. The two borders are anterior or cephalic and posterior 

 or caudal (Fig. 233). It is generally held that the lateral ridge, 

 of which the limb buds are specialized parts, represents a 



