PHYLUM CHORDATA 



57 



late period, so that the neurocoele opens externally in front by a 

 wide aperture, the neuropore (Figs. 761, 762, and 763, np). 



While the central nervous system is thus being formed, the 

 endodermal wall of the archenteron develops dorso-laterally a pair 

 of longitudinal folds (* in Fig. 760, A and B), the cavities in which, 

 continuous with the archenteron, are the beginnings of the entero- 

 coelic system. Transverse folds which appear divide the longitudinal 

 folds into segments, with the result that the archenteron comes to 

 have appended to it dorso-laterally a paired series of offshoots, the 

 enteroccelic or coelomic pouches (Fig. 760, 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 



US /l 



Flo. 761. Amphioxus lanceolatus. Embryo. A, from tue side; , in horizontal 

 section, ak. ectoderm : CM. neurenteric canal ; dk. archenteron ; ik. endoderm ; mk. meso- 

 dermal folds ; n. neural tube ; ud. archenteron ; us. first ccelomic pouch ; ush. ccelomic 

 cavity ; V. anterior ; H. posterior end. (From Korschelt and Heider, after Hatschek.) 



existence. Before long the coelomic pouches separate from the 

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

 or somites (Fig. 760, C, D), lying between ectoderm and endoderm. 

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

 cavities uniting become the coelome, which is therefore an enterocoele, 

 like that of Sagitta and the Echinodermata. 



While the coelomic sacs are in course of formation a median groove 

 appears along the dorsal wall of the archenteron (Fig. 760. B, C, ch) : 

 it deepens, loses its tubular character, and becomes a solid rod, the 

 notochord (D, ch), lying immediately beneath the nerve-tube. The 

 ordinary endoderm cells soon unite beneath it and so shut it off 

 from the archenteron. It will be seen that the notochord, like the 



