548 CEPHALOCHORDA. 



The principal change to be noted in the notochord is the great 

 development of its anterior end (Figs. 287, 288). It now extends 

 to the anterior end of the body, a feature in which Ampkioxus is 

 distinguished both from the Tunicata and from the Vertebrata. We 

 have already mentioned that the posterior end of the chorda-rudiment 

 in later stages becomes abstricted from the enteric wall in the same 

 way as the mesoderm-folds, and is then independent. In the mean- 

 time, histological differentiations take place which foreshadow the 



mr rh. 



' I C 



f- : ' < ' - ■ ,'-=>-->- 7.-7- - ~S 



'd 



en 



B 



4U^ 





v 



r so 



\ nrr 



d 



-■*' SI) 



Fig. 288.— A, larva of A mphioxus with the rudiment of the oral aperture and the first 

 gill-cleft, seen from the left .side (after Hatschek) ; B, anterior end of the same 

 larva, highly magnified, c, larval caudal tin; ch, chorda; en, neurenteric canal ; 

 d, alimentary canal : h, cavity caused by the transformation of the right anterior 

 entoderm-sac ; /.. club-shaped gland ; //. efferent portion of the same; ks, gill-cleft; 

 m, mouth ; mr, medullary tube ; np, neuropore ; sv, sub-intestinal vein ; w, ciliated 

 organ (pre-oral pit). 



final condition of the chorda. In transverse section the chorda is 

 seen to be composed of about four cells one above the other (Fig. 

 284). Small, round vacuoles now appear in the protoplasm of these 

 cells. These vacuoles in the uppermost and undermost rows of cells 

 remain small, but in the two middle rows they run together to form 

 large vacuoles, the order at the same time becoming changed in such 

 a way that a large vacuole compressed from before backward alternates 

 with a cell. The cells of the middle layers then form partitions 

 between the successive large vacuoles. A similar stage of develop- 

 ment was described above for the chorda of the Ascidians. According 

 to Lwoff's researches (No. 16), the chorda-tissue' of Amphioxw 



