XIII 



PHYLUM CHORDATA 



33 



blastopore. In this process of closing-in of the medullary groove 

 the fold which passes round behind the blastopore takes an im- 

 p o r t a n t part, 



growing forwards ^\ 



over the pos- 

 terior portion of 

 the canal. The 

 blastopore, thus nolo 

 enclosed in the ^ 



medullary canal, 

 persists for a 

 time as a small 



p e n i n g t h e 

 neurenteric canal 

 by which the 

 neuroccele and 

 enteric cavity are 

 placed in com- 

 munication. At 

 the anterior end 

 of the medullary 

 canal, owing to 



1 t s incomplete 

 closure in this 

 region, there re- 

 mains for a time 

 an opening the 

 neuropore (Fig. 

 743, neur.) 

 leading to the 

 exterior. 



A notochord 

 (Figs. 742, 0, A 

 743 and 744, 

 noto.) is formed 

 from certain of 

 the cells of the 

 wall of the arch- 

 enteron along the 

 middle line of the 

 dorsal side. 

 These are 

 arranged to form 

 an elongated cord 

 of cells which 



becomes completely constricted off from the endoderm of the wall 

 of the archenteron, and comes to lie between the latter and the 



VOL. II D 



end, 



FIG. 743. Later stages in the development of Clavellina. A, 

 approximately median optical section of a larva in which the 

 medullary canal (neurocoele) has become enclosed throughout, 

 communicating with the exterior only by the neuropore at the 

 anterior end and with the archenteron by the neurenteric canal ; 

 B, larva with a distinct rudiment of the tail and well-formed 

 mesoderm-layer and notochord. Letters as in preceding figure ; 

 in addition, mes. mesoderm. (After Van Beneden and Julin.) 



