422 



CEPBALOCHORDA. 



At this stage the larva is much more active than the 

 adult. 



The later larvse are more sedentary, lying much on the 

 right side, and they are strongly asymmetrical. The mouth 



Fig. 183. — Sections through embryos of Amphioxus, to 

 illustrate development of body cavity. 



On the upper line, three longitudinal sections ; on the lower line, 

 three transverse sections, ec. , Ectoderm; <?«., endoderm ; a., 

 archenteron ; p.s., primitive segments (protovertebrse) ; n.c, 

 nerve-cord;^., posterior end; np., neuropore ; ne.c, neuren- 

 teric canal; m.p., medullary or neural plate; ck., notochord ; 

 >/>., splanchnoccel, above it is the mvoccel. 



is placed at the left side ; the gill-slits of one side appear 

 considerably before those of the other ; the primitive seg- 

 ments of one side are not opposite those of the other, and 

 so on. By the process known as the " symmetrisation " of 

 the larva, the apparent symmetry of the adult is produced. 



