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 loAvest living Vertebrates (Amphioxus and 



I 

 I 



Fig. 120. — Transverse section through the pelvic region and the hind 

 Umbs of a Chick, on the fourth day of incubation (about 40 times the natura 

 size) : h, hoi'n-plate ; it', medullary tube ; n, spiual canal ; u, primitive kid- 

 neys ; X, chorda ; e, hind limbs ; b, allantois canal in the ventral vrall ; t, 

 aorta ; v, cai'dinal veins ; a, intestine ; d, intestinal-glandular layer ; /, in- 

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

 (cceloma). (After Waldeyer.) 



the Cyelostoma) 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 difterentl}' : in Fishes they 



