EMBRYOLOGY 



485 



and by backward outgrowths to spaces in the metapleural 

 folds. In the adult the^se latter spaces are represented by 

 lymph canals of doubtful origin. The walls of the spaces 

 give rise to mesodermal tissues around the mouth and in the 

 metapleural folds and to the first myomere. The remaining 

 mesoblastic somites all behave alike. They extend down- 

 wards on each side (Fig. 364) till they meet below the gut. 



Fig. 364. — Transverse sections of embryos of Amphioxus at successive 

 stages later than that represented by 14 (Fig. 361). — After Hatschek. 



ent.. Gut or enteron ; m.J>l., muscle plate; m.s., mesoblastic somite; myoc, 

 myoccele ; ».c, nerve cord; «.^>., neural plate; nek., notochord ; s£lc, 

 splanchnoccele. 



The outer or somatic wall of each lies against the epiblast, 

 together with which it is known as the somatopleure ; the 

 inner or splanchnic wall lies against the hypoblast and is 

 known with it as the splanchnopleure. The longitudinal 

 septum or ventral mesentery between the cavities of the 

 two sides now breaks down, so that they become con- 

 tinuous. Meanwhile there has formed in each of them a 

 horizontal septum which divides it into a dorsal half and 



