44 OUTLINES OF CHORDATE DEVELOPMENT 



B), pass around the pharyngeal wall to the dorsal side, and 

 there extend backward as a pair of ciliated bands which later 

 fuse into a single median structure the epipharyngeal or 

 hyperbranchial groove. 



5. The Month and Associated Structures 



This opening on the left side of the head continues to en- 

 large during the early part of the larval period, and finally 

 extends from a point considerably in front of the first gill slit 

 to the region of the fifth gill slit (Fig. 14). When the second- 

 ary slits begin to develop and the pharynx rotates back to a 

 normal position, the mouth gradually shifts its position in a 

 horizontal plane, moving around to the anterior end: it finally 

 reaches an anterior median position at about the critical 

 stage (Fig. 15). As the mouth migrates its margin extends 

 inward as the velum, like that of Ammocoetes, finally reducing 

 the mouth opening to a small aperture. Later this becomes 

 fringed with outgrowths from the free margin of the velum- 

 the velar tentacles (Figs. 15, 16). From the base of the velum 

 the oral cirri grow out. These are first indicated about the 

 time the secondary gill slits appear, and by the time of the 

 critical stage they are well developed and their cartilaginous 

 supports have become differentiated. At the same time the 

 velum begins to form, folds of the integument, first above 

 and then below r the mouth, grow out, the latter passing for- 

 ward toward the region of the snout. These folds are the 

 rudiments of the oral hood, and as the mouth passes forward 

 they enlarge and shift so as to form the left and right sides 

 respectively of the oral hood. The right fold becomes contin- 

 uous with the extremity of the dorsal fin as this turns the 

 anterior end of the larva (Figs. 15, 16). 



6. The Preoral Pit and Its Derivatives 



The preoral pit, the formation of which from the left anterior 

 gut diverticulum has been described, was left as a small sac 



