56o 



NOTOCHORD 



plate has become separated from the outer ectoderm. 

 In the mid-dorsal line a thickening of the endoderm 

 {ch) soon becomes constricted off to form the notochord 



A R 



/•'■ — 



Fig. 143. — Four stages in flie Jevelopment of the notochord, nervous system, and 



mesoderm o{ Aviphioxus. 



ak. ectoderm ; ch notochord ; dh. cavity of aichenteron ; hb. ridge of ectoderm 



growing over medullary plate ; ?/fe, endoderm ; Ih. coslome ; ink. coelomic pouch ; 



mk\, parietal layer of mesod'erm ; ink'^. visceral layer ; mp. medullary plate ; «. 



medullary cord ; ns. proto vertebra. (From Korschelt and Heider, after Hatschek.) 



(pp. 203, 404 and 425), and on either side of this 

 a series of hollow endodermic pouches arise, arranged 

 metamerically (Figs. 143, 7nk^ and 144, us^ 7?ik). The 



