SOMITES. 



89 



ventral side of the alimentary canal. The septum between them 

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

 sides become contmuous. 





i^:fc;si^ve--^-^^T|^ - > 



1_- 



dc is 1 



'°- f*-~''-'"'o ''"■val stages of ilmp/itoTKi from the right side, showing the origin of the cill- 

 clefis of tlie rigtit side of the a.Iult (from Korschelt and Heider. after Wiiley) 1 2 

 9, 12, 14 first, etc., to fourteenth gUl-cleft of future left side • /, VH first and seventh 

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

 chord ; dr club-shaped glani ; es rudiment of endostvle ; fh dorsal, fh' ventral fln spaces • 

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



metapleure : 

 preoral pit. 



n nerve cord ; p atrial cavity ; «i sub-pharyngeal vessal ; v velum 



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

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

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

 side ; be buccal cirri ; ch notochord ; es endostvle ; 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 



