28 STRUCTURE OF THE VERTEBRATES 



cavity. Continue the multiplication of windows, and eventually 

 the internal cavity remains only in the region above the win- 

 dows, as narrow strips between them, and ventral to the inner 

 cavity. As the gill slits of Amphioxus are very long and close 

 together the coelomic cavity of the adult is left (1) as a ven- 

 tral tube; (2) two dorsal tubes separated by the mesentery; 

 and (3) long, very narrow tubes, between the primary gills, 

 which connect the dorsal and ventral portions. During develop- 

 ment each of these primary gills is divided into two by the 

 downgrowth of a septum which naturally does not carry any 

 of the coelomic cavity with it. Therefore, only every alternate 

 gill support has a tube of coelom. 



The rostrum and the anterior extension of the notochord are 

 also considered as specializations. This is based on the fact that 

 in all vertebrates the notochord extends no farther than the 

 middle of the brain, and in Amphioxus the rostrum is a later 

 growth. Unfortunately the notochord of the more primitive chor- 

 dates offers no solution, for in the tunicates the notochord is 

 limited to the tail; and in the more primitive hemichordates the 

 notochord is rudimentary and found in the anterior region. 



The asymmetry of the mouth opening is correlated with a 

 bilateral slippage during development. One half of the embryo 

 pushes forward about half the distance of one body segment, 

 and though the mouth finally corrects its position, the myotomes 

 and the gonads maintain this asymmetry throughout life. This 

 accounts for the fact that when cross sections of Amphioxus are 

 studied a pair of gonads is never cut through the same region, 

 one section always being larger than the other. 



