30 OUTLINES OF CHORDATE DEVELOPMENT 



of the endodermal wall of the archenteron. At the close of the 

 embryonic period (Fig. 9, C) fourteen or fifteen pairs of somites 

 have been formed, and posterior to these remain short undivided 

 regions of mesoderm from which additional somites will be 

 formed during the larval period. These somites are of course 

 not all in the same stage of development, but passing along the 

 series in the anterior direction they rep- 

 resent successively more advanced con- 

 ditions. Advance is indicated by in- 

 crease in the size of the cavity of the 

 somite, thinning of the wall, and the 

 ventral and somewhat posterior exten- 

 sion of the somite between the wall of 

 the gut and the superficial ectoderm 

 (Fig. 8, E). By the end of the embry- 

 onic period the more anterior pairs are 

 already bent into the < -form character- 

 istic of the adult. The first pair of 

 somites send forward unusual extensions 

 from the antero-dorsal region along each 

 side of the notochord (Fig. 9, B) : it is 



ssible ' thou s h hard 'y like] y> that these 



larva with six pairs of indicate the former presence of an addi- 



mesodermal somites, at ,. -, f ., . .-, . ., 



the level of the notochord tional pair of somites in this region: it 

 and somites. After Cer- see ms more likely that the condition is 



fontaine. a, Archenteron; 



e, enteroccei;w, notochord; the result of the anterior prolongation of 



d '/If r s l a ite S . SiXth meS - the notochord beyond the proper region 



of the first somite. The walls of these 



extensions later go through the same developmental history 

 as those of a typical somite. 



By the time five or six pairs of somites are formed they begin 

 to show that bilateral alternation which is so characteristic of 

 the adult (Fig. 10). Only the first and the upper part of the 

 second pairs of somites lie exactly opposite. In the other pairs 

 the left member comes to lie in advance of the right, and farther 

 posteriorly the segments of the two sides exactly alternate. 

 It is also noticeable that the left side develops slightly in advance 



