122 



DEVELOPMENT OF AMPHIOXUS. 



-n c 



~s. t. tf 



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 becdmes a continuous unseg- 



mented cavity. On account 

 of the fact that the archen- 

 teric pouches give rise both 

 to the cavity of the somites 

 (myoccel} and to the general 

 body-cavity (coelom proper 

 or splancJinoccel}, they are 

 often spoken of as the myo- 

 ccelomic pouches. The cav- 

 ity of the original archen- 



Fig. 67. Scheme of a transverse teric pouches is known as 



section through the body of a larva t , primitive ffflnni the 



with five gill-slits, to show the division "* ;/ ' LU 



between myoccel and splanchnoccel. epithelial walls of which 

 (After HATSCHEK.) 



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



^Notochord. l.m. Muscles. myMyo- Ag d iff erent iation and OF- 



coel. sf. Rudiment of sclerotome. 



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



vein. sp. Splanchnoccel. 



derm gives rise to (i) the 



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

 (4) the reproductive 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 yet 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 



