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. 



I^EirsMif^ 





dr es 1 



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 • I, VII first ana 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 ; fk dorsal, fh' ventral fin spaces ; 

 k rudiments of the later form.ed gUl-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 ; /, VIII first and eighth of the later formed clefts of the right 

 side ; be buccal cirri ; eh 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 



