PRIMITIVE VERTEBRAE. 289 



into five bladders: v, fore-brain; z, twixt-brain; ra, mid-brain; h, hind- 

 brain ; n, after-brain ; a, eye-vesicles ; g, ear-vesicles ; c, heart ; dv, yelk- 

 veins ; rap, marrow-plate ; uw, primitive vertebra. 



developed from an inarticulate worm-form by terminal 

 budding, so the many-membered vertebrate body has 

 originated from an inarticulate parent-form. The nearest 

 extant allies of. this parent-form are the Appendicularia 

 (Fig. 162) and the Ascidian (Plate XL Fig. 14). 



As has been repeatedly pointed out, this primitive 

 vertebral, or metameric structure has a very important 

 bearing on the higher morphological and physiological de- 

 velopment of Vertebrates. (Cf. vol. i. p. 346.) For the articu- 

 lation is by no means confined to the vertebral column, but 

 equally affects the muscular, nervous, vascular, and other 

 systems. As is shown by the Amphioxus, the metameric 

 structure appeared much earlier in the muscular than in 

 the skeleton system. Each so-called primitive vertebra is 

 in fact far more than the mere rudiment of a future verte- 

 bra. In each primitive vertebra exists the rudiment of a 

 segment of the dorsal muscles, of a pair of spinal nerve- 

 roots, etc. Only the inner portion — that which lies directly 

 next to the notochord and the medullary tube — is employed, 

 as the skeleton-plate, in the formation of actual vertebrae. 

 We have already seen how these true vertebras develop from 

 the skeleton-plate of the primitive vertebras or metamera. 

 The right and left lateral halves of each primitive vertebra, 

 originally separate, unite. The ventral edges, meeting below 

 the medullary tube, surround the chord and thus form the 

 rudiments of the vertebral bodies ; the dorsal edges, meeting 

 above the medullary tube, form the first rudiments of the 

 vertebral arches. (Cf. Figs. 95-98, and Plate IV. Figs. 3-8.) 



