SOMITES. 



39 



ventral side of the alimentary canal. The septum between them 

 (ventral mesentery) breaks down and the somites of the two 

 sides become continuous. 



FIG. 24. Two larval stages of Amphioxus from the right side, showing the origin of the gill- 

 clefts of the right side of the adult (from Korschelt and Heider, after Willey). 1. 2, 

 9, 12, 14 first, etc., to fourteenth gill-cleft of future left side /, VII first and seventh 

 of the later formed set which eventually belong to the right side ; au eye-spot ; ch noto- 

 chord ; dr club-shaped gland ; es rudiment of endostyle ; fh dorsal, /&' ventral fin spaces ; 

 k rudiments of the later formed gill-clefts ; m margin of mouth ; mf edge of right 

 metapleure ; n nerve cord ; p atrial cavity ; si sub-pharyngeal vessel ; ^v velum ; w 

 preoral pit. 



FIG. 25. Ventral view of Amphioxus larva, rather later than Fig. 24 (from Korschelt and 

 Heider, after Willey). 2 second, 12 vestige of twelfth cleft of the first-formed row, now 

 passed on to the left side ; I, VIII first and eighth of the later formed clefts of the right 

 side ; be buccal cirri ; ch notochord ; [es endostyle ; m mouth (larval) ; v velum. ; 



At the same time a septum is formed, dividing off the dorsal 

 part of the somites from the ventral. The dorsal part becomes 



