58 



ZOOLOGY 



SECT. 



a narrow cleft. The medullary folds extend behind the blastopore 

 so that, when they unite, the latter aperture opens into the 

 neurocoele by a neurentcric canal (Fig. 752, 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 nmvopore (Figs. 

 752, 753 and 754, njo). 



While the central nervous system is thus being formed, the 

 endoderm sends out dorsal ly a paired series of offshoots, the 



B 



mp — 



—mk 







D 



-//( 



ik 



Fig. 751.— Amphioxus lanceolatus. Four stages in the development of the notochord 

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

 hb, ridge of ectoderm growing over medullary plate ; il-, endoderm; Ik, ccelome; mk, ccelomic 

 Ijouch ; jKi-i, parietal layer of mesoderm; ink~, visceral layer ; mp. medullary plate ; n. neuron ; 

 m, protovertebra. (From Korschelt and Heider, after Hatschek.) 



ccelomic 2'>onchcs (Fig. 751, mlc), arranged metamericall^^ 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 ccelomic pouches separate from the 

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

 sacs (Fig. 751, C, D), lying between ectoderm and endoderm. 

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



