II. II 



DEVELOPMENT OF AMPHIOXUS 



43 



bours. The cells of each pouch push down ventrally on either side of 

 the gut, the outer ones applying themselves to the body wall to form 

 the somatopleure, the inner to the gut wall as splanchnopleure (Fig. 

 1 8 d). The inner wall of the mesoderm on either side of the nerve- 

 cord thickens to form the myotome, and a tongue of cells growing up 

 between this and the nerve-cord forms the sheaths of the latter and 



lOOfJL 



my coel. 



som.pl. 



A B C D 



Fig. i 8. Further stages in the development of amphioxus as seen in transverse 



sections. 

 A, stage of three somites; B, six somites; c, nine somites; D, eleven somites, arch, archenteron; 

 coel. coelom; tries, mesoderm; my. myotome; my. coel. myocoele; n. notochord; n.c. nerve-cord; 

 som.pl. somatopleure; spl.pl. splanchnopleure; spl.coel. splanchnic coelom. (After Hatschek.) 



neurp. 



g. ioo u. 



Fig. 19. Young amphioxus, soon after hatching. 

 g. gut; «. notochord; n.c. nerve-cord; nenr.c. neurenteric canal; neur.p. neuropore. 



probably also the fin-ray boxes and other 'mesenchymal' tissues. The 

 upper part of the coelomic cavity, the myocoele, becomes separated 

 from the ventral splanchnocoele. Whereas the former becomes almost 

 completely obliterated, the latter expands to form the adult coelom, 

 the cavities between the adjacent sacs breaking down. 



While this differentiation of the mesoderm has been proceeding the 

 animal has elongated into a definitely fish-like form. The neural tube 

 is a small dorsal canal, opening by an anterior neuropore and con- 

 tinuous behind through a neurenteric canal with the gut (Fig. 19). 

 The larva hatches when only two segments have been formed and 

 swims at the sea surface by means of its ciliated epidermis, turning on 

 its axis from right to left as it proceeds with the front end forwards. 



The mouth now appears as a circular opening and then moves 

 over to the left side and becomes very large. From this time onward 

 the whole development is markedly asymmetrical, presumably in 



