132 



of diverticula which are the enterocoeles or coelomic pouches, 

 and give rise to the mesoblastic somites (Fig. 84, B and C, ra&). 

 The notochord (Fig. 84, D, cli) is at first a longitudinal cellular 

 ridge, which becomes segmented off from the hypoblast as a rod 

 lying below the neural canal. It is seen in various stages of 

 A B 



FIG. 84. Four stages in the development of the notochord, nervous system, and meso- 

 derm of Amphioxus. ak, Ectoderm ; ch, notochord ; dh, cavity of archeuteron ; 

 hb, ridge of ectoderm growing over medullary plate; ik, endoderm ; Ih, coelom ; 

 mk, coelomic pouch ; mk l , parietal layer of mesoderm ; ink" 1 , visceral layer ; mp, 

 medullary plate ; n, neural canal ; ns, protovertebra. (From Korschelt and 

 Heider, after Hatschek.) 



development in Figs. 84 and 86, leading to the vacuolated con- 

 dition of the adult. 



The coelomic pouches are five in number (1) one median, 

 anterior, which gives rise to the two head cavities, the left-hand one 

 of which opens to the exterior by means of the pre-oral pit ; (2) 

 a pair of small lateral pouches, placed anteriorly and dorsally, which 

 do not divide but give rise to the first pair of myotomes only and 

 their outgrowths which extend back into the metapleural folds, 

 where, however, they are later replaced by lymph spaces ; and 



