124 DEVELOPMENT OF AMPHIOXUS. 



Origin of the Notochord. 



The notochord is formed from the endodermic cells 

 which lie between the mesodermic pouches and constitute 

 the dorsal wall of the archenteron. The dorsal wall of 

 the archenteron at an early stage becomes converted into 

 a shallow longitudinal groove whose concavity is turned 

 towards the archenteric cavity (Fig. 65 D). This groove 

 gradually deepens (Fig. 65 E), and eventually its walls 

 become closely appressed to one another so as to obliter- 

 ate the lumen (Fig. 65 F). Finally the adjoining cells of 

 the archenteric wall grow across the gap occasioned by 

 the formation of the notochord, and joining together, shut 

 off the latter from any participation in the enteric wall 

 (Fig. 66). In this way is the notochord separated from 

 the endoderm gradually from before backwards. Poste- 

 riorly it remains for a considerable time fused with the 

 endoderm at the point where the anterior wall of the neu- 

 renteric canal becomes continuous with the dorsal wall 

 of the archenteron. 



We have indicated above that the differentiation of the 

 notochord takes place from before backwards. At its 

 anterior extremity a very noteworthy exception to this 

 rule is presented. In the region of the first myotome 

 the notochord retains an open communication with the 

 archenteron after its lumen has already been obliterated 

 behind this point. Moreover, in the embryo, with eight 

 pairs of myocoelomic pouches (Fig. 68 bis), the front end 

 of the notochord lies some distance behind the front end 

 of the body, while the anterior portion of the archenteron 

 extends beyond the notochord. Eventually the notochord 

 is continued to the front end of the body by becoming 

 constricted off from the dorsal wall of the anterior sec- 



