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 neurenteric 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 neuropore (Figs. 

 752, 753 and 754, np). 



While the central nervous system is thus being formed, the 

 endoderm sends out dorsally a paired series of offshoots, the 



D 



nerv^ts^?.™ ?^H lanceolatus. Four stages to the development of the notochord 

 ^fiXe of lr?n,1p?™ '"'=^';'i<='™- «*. eotoderm ; rh, notochord ; dh, cavity of archeiiterou ; 

 Boudi- ™H nnrttn^r T *■ <=ndodevm ; I'h, coelome ; ml:, coelomio 



pouch mil parietal layer ^f mesoderm; mt2, visceral layer ; mp. mediilarv plate • n neuron ■ 

 71S, protovertebra. From Korschelt and Heider, after Hatschek ) ' ' 



caslomic poiiches (Fig. 751, 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 ccelomic 

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

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



