XIII 



PHYLUM CHORDATA 



53 



cavities become the coelome, which is therefore an enteroccele, 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. 706, 

 B, C, c$) : it deepens, loses its tubular character, and becomes a 

 solid rod, the notochwd (D, ck\ 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 neuron, never exhibits any trace of seg- 

 mentation. At its first formation it stops short of the anterior 



VK.. 707. Amphioxus lanceolatus. Embryo. A, from the side ; B, in horizontal 

 section, cd, ectoderm ; en, neurenteric canal ; dh, archenteron ; ik, endoderm ; ml-, meso- 

 dermal folds ; w /neural tube ; v.if, archenteron ; us, first ccelomic pouch ; ush, coelomic cavity ; 

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



end of the archenteron : its final extension to the end of the 

 snout is a subsequent process. 



New coslomic pouches are formed in regular order from before 

 backwards, the embryo at the same time elongating and becoming 

 laterally compressed and pointed fore and aft. At the anterior end 

 the mouth (Fig. 708, m) appears on the left side of the body as a 

 small aperture, which soon increases greatly in size. On the ventral 

 surface another small aperture, the first gill-slit (ks) makes its 

 appearance, and soon shifts over to the right side : it forms a 

 direct communication between the pharynx and the exterior, 

 like the stigmata of Appendicularia (p. 21) : there is at present no 

 trace of the atrium. 



The anterior end of the archenteron has meanwhile grown out 



