OEIGIN 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 natura 

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

 neys ; 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 ; g, germ-epithelium ; r, dorsal muscles ; c, body-cavity 

 (cceloma). (After Waldeyer.) 



the Cyclostoma) are at the present time. The descendants 

 of these primseval, 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 



