PHYLUM CHORDATA 



59 



cavities become the ccslomo, which is therefore an enteroccele, like 

 that of Sagitta and the Echinodermata. 



While the ccslomic sacs are in course of formation a median 

 groove appears along the dorsal wall of the archenteron (Fig. 751, 

 B, C, ck) : 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 neuron, never exhibits any trace of seg- 

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



Fig. 752.— Ampbioxus lanceolatus. Embryo. A, from the side; B, in horizontal 

 section, ak, ectoderm ; «t, neurenteric canal ; dh, archenteron ; ik, endoderm ; mfc, meso- 

 dermal folds ; n, neural tube ; -ud, archenteron ; us, first crelomic pouch ; ush, coelomic cavity ; 

 V, anterior ; S, posterior end. (From Korschelt and Heider, after Ilatschek.) 



end of the archenteron : its final 

 snout is a subsequent process. 



extension to the end of the 



The significance of these early stages in the development of Amphioxus has 

 been variously regarded by different embryologists, and it is impossible to give 

 here more than a statement of the ascertained facts, leaving the question of 

 their interpretation, which can only be profitably discussed on the broadest basis, 

 as a matter for more advanced study. Since, however, the gastrulation and 

 mesoderm-formation in Amphioxus are, almost universally, made the starting- 

 point in the process of interpreting the phenomena of early development in all 

 Vertebrates, it may be desirable at the present stage to state that though in the 

 foregoing account the cells which are invaginated are referred to throughout 

 simply as endoderm, this view of their nature is not the only one that may 

 reasonably be held. If the inner layer of the gastrula be composed of endoderm 

 and of endoderm only, certain consequences necessarily follow ■ the notochord 



