THE DEVELOPMENT OF AMPHIOXUS 



25 



their dorsal walls. The first pair of the transverse folds 

 appears a short distance back from the anterior ends of the 



n ch 



eg co 



n P cv 



rd 



eg gs 



spc 



FIG. 9. Optical sections of young embryos of Amphioxus. After Hatschek. 

 The cilia are omitted. A. Two-somite stage, approximately at the time of 

 hatching, showing relation of neuropore and neurenteric canal. B. Nine- 

 somite stage, showing origin of anterior gut diverticu-la. C. Fifteen-somite 

 stage. End of the embryonic period, ap, Anterior process of first somite ; c, 

 neurenteric canal; ch, notochord (or its rudiment, in ^4.); eg, club-shaped gland 

 (or its rudiment, in B); ego, external opening of club-shaped gland; co, ccelomic 

 cavity of somite; cv, cerebral vesicle; g, gut cavity (enteron, mesenteron) ; gs, 

 rudiment of first gill slit; i, intestine; Id, left anterior gut diverticulum (preoral 

 pit in C) ; m, mouth; mes, unsegmented mesoderm; n, nerve cord (or its rudiment, 

 in A) ; np, neuropore; p, pigment spot in nerve cord; rd, right anterior gut diver- 

 ticulum (preoral head cavity in C) ; si, S2, first and second mesodermal somites; 

 spc, splanchnoccel (body cavity). 



mesoderm folds and soon cuts off this region, forming thus 

 the first pair of mesodermal somites, as they are called. The 



