32 OUTLINES OF CHORDATE DEVELOPMENT 



cavities are completely separated. By the close of the embry- 

 onic period the anterior and posterior walls of the somites in the 

 region of the splanchnoccels also break through, leaving these 

 cavities continuous antero-posteriorly as well as transversely. 

 The unsegmented cavity thus formed is the ccelomic cavity of 



up 



FIG. 11. Sections through young Amphioxus embryos showing the origin of 

 the anterior gut diverticula. After Hatschek. The cilia are omitted. A. 

 Frontal section through embryo with nine pairs of somites. (See Fig. 9, 5). 

 The dotted line marks the course of the gut wall ventral to the level of the section. 

 B. Optical sagittal section through anterior end of embryo with thirteen pairs 

 of somites, showing position of right anterior gut diverticulum. C. Same in 

 ventral view, c, Ccelomic cavity of somite; ch, notochord; csg, rudiment of 

 club-shaped gland; d, rudiment of anterior gut diverticula; ec, ectoderm; en, 

 endoderm; g, gut cavity (enteron, mesenteron); gsl, rudiment of first gill slit; 

 Id, left anterior gut diverticulum; n, nerve cord; np, neuropore; rd, right anterior 

 gut diverticulum; si, $2, sg, first, second, and ninth mesodermal somites. 



the embryo and larva which is seen to be a true synccelom 

 (Fig. 19, A). 



There remain to be described, in connection with the develop- 

 ment of the mesoderm, a pair of outgrowths from the anterior 

 end of the enteron in front of the first pair of mesodermal so- 

 mites. These cavities are known as the anterior gut diverticula 



