THE DEVELOPMENT OF AMPHIOXUS 23 



the neural folds. In some individuals, and these have usually, 

 and erroneously, been described as the more typical, the folds 

 meet first over the blastopore and gradually fuse thence ante- 

 riorly. The more frequent relation seems to be that just 

 described, and it is important to recognize that this agrees 

 with the method of closure of the neural folds in practically 

 all of the Craniates, where they meet and fuse first in the 

 middle or anterior to the middle of the embryo, and then fuse 

 in each direction from that region. Amphioxus differs, how- 

 ever, from all other Chordates in that the margins of the neural 

 plate do not remain connected with the neural folds for a time, 

 and are not elevated and closed into a tube at the same time 

 the neural folds close together: the folding of the neural 

 plate into a tube will be described presently. 



5. The Formation of the Notochord 



The chorda develops more slowly than the nervous system 

 and by the time of hatching has hardly more than commenced 

 its formation. The rudiment of the notochord is a median 

 strip of endoderm, six to nine cells wide, forming the roof of the 

 archenteron and lying consequently just beneath the neural 

 plate and in contact with its lower surface (Fig. 8, B}. The 

 depression of the neural plate depresses also the flattened 

 dorsal wall of the archenteron, so that the chorda rudiment 

 pushes down into the archenteric cavity and appears in section 

 concavely arched. The chorda cells are those inflected at 

 the dorsal margin of the blastopore together with their descend- 

 ants. While the gastrula or embryo is elongating the rudiment 

 of the notochord divides posteriorly, passing a short distance 

 around each side of the blastopore, and terminates in a growth 

 zone similar to that concerned in the extension of the neural 

 plate but lying just inside instead of outside the blastoporal 

 rim (Fig. 9, B). At the time of hatching the rudiment of the 

 chorda still remains as a flat plate of cells directly continuous, 

 laterally and anteriorly, with the endoderm lining the remainder 

 of the archenteron. 



