XIII 



PHYLUM CHORDATA 



33 



mes 



noto 



neur- 



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 * A 



over the pos- 

 terior portion of 

 the canal. The 

 blastopore, thus 

 enclosed in the 

 medullary canal, 

 persists for a 

 time as a small 

 o 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 

 its 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, G, D, 

 743 and 744, 

 noto.) is formed 

 from certain of 

 the cells of the 

 wall of the arch- 

 enteron along the 

 middle line of the 



end 



side. 



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

 approximately median optical section of a larva in which the 

 medullary canal (neuroccele) 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-Iayer and notochord. Letters as in preceding figure ; 

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



dorsal 



These are 



arranged to form 



an elongated cord 



of cells which 



becomes completely constricted off from the endoderrn of the wall 



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



VOL. II D 



