DEVELOPMENT OF AMPHIOXUS 



107 



the outside at the bottom of an ectodermal inpushing called 

 the preoral pit ; in the adult this opening is Hatschek's pit. 



There is no mesenchyme in Amphioxus, and the connective 

 tissue which surrounds the nerve-tube and notochord is derived 

 from hollow ingrowths from the myocoels, forming the 

 scleroccels, the walls of which are the sclerotomes. The fin-ray 

 boxes are also nipped off from the myoccels. 



Portions of the myoccel persist in the adult between the 

 myotomes and the connective tissue which surrounds them. 

 Lastly, a downgrowth from each of the myoccels in the anterior 



. 9. 



B 3 



Fig. 70. — Amphioxus : young embryo and larva. 



A, seen from above ; B, seen from the left side, a, anterior-gut diverti- 

 cula ; g, gut ; ?i, notochord ; nc, nerve-cord ; s, mesodermal somites. 



region of the body gives rise to the gonocoels, the walls of which 

 (gonotomes) give rise to the gonads. 



It is important to notice that the whole of the mesoderm in 

 Amphioxus is segmented, and that this segmentation is 

 retained everywhere except in the region of the splanchnoccel. 



The Gut. — At the stage when there are two pairs of somites 

 nipped off, the embryo hatches and emerges from the vitelline 

 membrane as a larva. The gut is still a closed sac which 

 communicates only with the nerve-tube, through the neuren- 

 teric canal. The mouth forms on the left side by a perforation 

 between the ectoderm and the endoderm immediately under- 

 lying it. It is very asymmetrical and soon becomes a large 

 opening bordered with cilia. In a similar way, the anus 



