66 



AMPHIOXUS. 



archenteron. and separate completely from this (Fig. 29). This 

 separation rapidly extends backwards, involving the hinder 

 somites in succession; and the somites now form (Figs. 27, 29) 

 a series of squarish hollow bodies, arranged in a row along each 

 side of the embryo, at the level of the notochord. 



The somites are at first small, and lie above or dorsal to the 

 alimentary canal (Fig. 29) ; but they rapidly increase in size, 

 and, extending ventral wards (Figs. 30 and 32), make their way 



NR 



-CH 



S 



NT 



FIG. 30. 



FIG. 31. 



FiGS. 30 and 31. Amphioxus embryos with nine pairs of mesoblastio 

 somites, x 224. (After Hatschek.) 



Fig. 30. Embryo seen in optical section from the right side. Fig. 31. Embryo seen 

 in horizontal section, at the level of the notochord. CH, notochord. DL, left anterior 

 gut diverticulum. DR., right anterior gut diverticulum. WE., neuropore. ~NT, neur- 

 enteric canal. PC, polar mesoblast cell. SI, first mesoblastic somite of the right side. 

 S9, ninth mesoblastic somite of the right side. T, mesenteron. 



round the sides of the embryo, between the gut wall and the 

 external epiblast, ultimately reaching the mid-ventral line, where 

 the somites of the right and left sides of the body become 

 continuous with one another. 



During their earlier stages (Figs. 27, 30), the long axes of 

 the somites lie transversely, or slightly obliquely to the axis of 

 the embryo ; but towards the close of the embryonic period 



