122 



DEVELOPMENT OF AMPHIOXUS. 



visceral walls of the cavity, at about the level of the base 

 of the notochord (Fig. 67). 



The dissepiments between the myotomes are formed 

 from the contiguous walls of the successive pouches, but 

 ventrally, in the region of the coelom, they break down, 

 so that the latter then becomes a continuous unseg- 



mented cavity. On account 

 of the fact that the archen- 

 teric pouches give rise both 

 to the cavity of the somites 

 {niyoccel) and to the general 

 body-cavity (coelom proper 

 or splanchnoccel), they are 

 often spoken of as the myo- 

 ccelomic pouches. The cav- 

 ity of the original archen- 



Fig. 67. — Scheme of a transverse tCric pOUcheS is knOWn aS 

 section through the body of a larva ,.v„ >,•■..■ „; ^u 



with five g,ii-siits, to .show the division ^^^ prwutive ccslom, the 



between myoccel and splanchnoco:!. epithelial walls of which 



(After Hatschek.) 



n.c. Spinal cord (medullary tube). Constitute the mesoderm. 



ch. Notochord. Lm. Muscles, wj'. iVIyo- a .., J'jx *.• *-• j 



, D r . f 1 , As differentiation and or- 



ccel. sc. Rudiment of sclerotome. 



al. Alimentary canal, s.i.v. Sub-intestinal ganogeny proceed, the mCSO- 



vein. sp, Splanchnoccel. 



derm gives rise to (i) the 

 miisculatiire, (2) the connective tissue, (3) the blood-vessels, 

 (4) the reprodtictive organs, (5) the ccelomic epithelium or 

 lining of body-cavity, also called the peritoneum, and 

 (6) the excretory tubules. The development of the last- 

 named structures has, however, not 3'et been worked out 

 in Amphioxus. 



The parietal layer of the mesoderm applies itself closely 

 against the ectoderm, and gives rise to the cutis of the 

 body-wall. 



The connective tissue-sheath of the notochord and 



