XIII 



PHYLUM CHORDATA 



59 



cavities become the ccclome, which is therefore an enterocadc, like 

 that of" Sagitta and the Echinoderniata. 



While the coelomic sacs are in course of formation a median 

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

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

 solid rod, the noiochord (D, ch), lying innnediately 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 



V 



-Ush 



iLsh 



Fui. 75-2.— Amphioxus lanceolatUS. Embryo. A, from the side ; B, in Imrizontal 

 section, a/c, ectoderm ; ca, neurentcric canal ; dh, :irchcnteron ; ilc, endoderm ; vih, meso- 

 dermal folds ; n, neural tube ; ud, archentoron ; j!.s, first Cfelornic pouch ; a-^h, cujlomic cavity ; 

 F, anterior; //, posterior end. (From Korschclt and Ileider, after Hatschek.) 



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

 snout is a subsequent process. 



The significance of these eai-ly stages in the development of Amphioxus has 

 l)een variously regai'ded by different embiyologists, 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 



