ORIGIN OF THE LIMBS. 



359 



not appear till later. This also is a fact of the profoundest 

 interest ; for it tells us that the older Vertebrates were 

 footless, as the lowest living Vertebrates (Amphioxus and 



FIG. 120. Transverse section through the pelvic region and the hind 

 limbs of a Chick, on the fourth day of incubation (about 40 times the natural 

 size) : h, horn-plate ; w, medullary tube ; n, spinal canal ; 11, primitive kid- 

 neya j x, chorda ; e, hind limbs ; b, allantois canal in the ventral wall ; t, 

 aorta; v, cardinal veins; a, intestine; d, intestinal-glandular layer; /, in- 

 testinal-fibrous layer ; <7, germ-epithelium ; r, -dorsal muscles; c, body-cavity 

 (c&loma). (After Waldeyer.) 



the Cyclostoma) are at the present time. The descendants 

 of these primaeval, footless Vertebrates did not acquire limbs 

 till a much later period in the course of their development, 

 when they acquired four limbs a front pair and a hind pair. 

 These limbs are all originally formed after one model, though 

 they afterwards develop very differently : in Fishes they 



