66 



AMPHIOXUS. 



archenteron, and separate completely from tliis (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. 



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

 a limentaryjj anal (Fig. 29) ; but they rapidly increase in size, 

 and, exten ding ventralwards (Figs. 30 and 32), make their way 



Fig. 30. 



Fig. 31. 



Figs. 30 and 31. — Amphioxus embryos with nine pairs of mesoblastic 

 somites, x 224. (After Hatsohek.) 



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

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

 gut diTertioulum. DR, right anterior gut diverticulum. HE,, ueuropore.' M"T, 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 th e sides of the embryo, between the gut wall and the 

 external epiblast, ultima tely reaching the mid-ventral line-where 

 the somites of the right a nd lelt sides ot the bod y become 

 ■con tinuous with, o ne anotiier. 



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 



