xm 



PHYLUM CHORDATA 



67 



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 fre3 



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

 section, ak. ectoderm ; en. ueurenteric 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 Helder, 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 ccelome, which is therefore an enterocosle, 

 like that of Sagitta and the Echinodermata. 



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

 appears along the dorsal wall of the archenteron (Fig. 760, B, G, 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 



