52 



ZOOLOGY 



SECT. 



enclosing a central canal, the ne-urocaele, continued dorsally into 

 a narrow cleft. The medullary folds extend behind the blastopore 

 so that when they unite the latter aperture opens into the 

 neurocoele by a neurenteric canal (Fig. 707, A, en). Anteriorly 

 the folds remain apart up to a late period so that the neurocoele 

 opens externally in front by a wide aperture, the neuropore (Figs. 

 707, 708 and 709, np).' 



While the central nervous system is thus being formed, the 

 endoderm sends out dorsally a paired series of offshoots, the 



D 



hb 



FIG 706. Amphioxus lanceolatus. Four stages in the development of the notochord 

 nervous system, and mesoderm. ak, ectoderm ; ch, notochord ; dh, cavity of archenteron ; 

 hb, ridge of ectoderm growing over medullary plate ; ik, endoderm ; Hi, coeleme ; mk, coelomic 

 pouch ; mk 1 , parietal layer of mesoderm ; mk'2, visceral layer ; nip. medullary plate ; n. nerve- 

 tube ; ns, protovertebra. (From Korschelt and Heider, after Hatschek.) 



ccelomic pouches (Fig. 706, mk) arranged metamerically. In this 

 way segmentation is established, and it is at this period that the 

 embryo ruptures its containing membrane and begins free 

 existence. Before long the coelomic pouches separate from the 

 archenteron and take on the form of a series of closed coelomic 

 sacs (Fig. 706, C, D), lying between ectoderm and endoderm. 

 From the walls of these sacs the mesoderm is derived, their 



