l6o DEVELOPMENT OF AMPIIWXUS. 



ectoderm and fuses with it, and then at the point of fusion 

 a perforation tal^es place, and the mouth is established in 

 the mid-dorsal line. During the formation of the mouth 

 the neuropore temporarily closes up, but subsequently it 

 reopens — into the nioittli. 



In Amphioxus we can only assume that in correlation 

 with the forward extension of the notochord, the mouth 

 was compelled to forsake its primitive relations to the 

 neuropore and to move to one side so as to make way for 

 the notochord. The growth of the latter to the front end 

 of the body obviously prevents the wall of the phar)-nx 

 from coming into contact with the ectoderm in the mid- 

 dorsal line, while it leaves the neuropore unaffected, since 

 the nerve-tube is essentially dorsal to the notochord, and 

 the pharynx, on the other hand, essentially ventral to it. 



This explains the fact that the hypophysis (olfactory pit) 

 of Amphioxus opens dorsally directly to the exterior instead 

 of into the mouth as it does in the Ascidian. 



The secondary gill-slits — that is, those belonging to the 

 primitive right side of the body — present an interesting 

 instance of retarded or latent development. This is due 

 to the fact that their own side of the body is at first 

 usurped by their primitive antimeres, the so-called primary 

 slits, as a result of which they have themselves been 

 temporarily crowded out as mentioned above. In con- 

 sequence of their retardation, when they do appear to 

 inaugurate the process of symmetrisation, they do not 

 conform to the method in which metameric structures are 

 normally produced, but most of them — namely, from the 

 second to the seventh inclusive — arise simultaneously 

 while the first and the eighth arise somewhat later. 



