44 



CHORDATE ORGANIZATION 



ii. ii 



connexion with the spiral movement and method of feeding. The first 

 gill-slit also forms near the midline but moves up on to the right side 

 (Fig. 20). At about the same time the right side of the pharyngeal wall 

 develops into a V-shaped thickening, the endostyle. Behind this there 

 forms a tube, the club-shaped gland, joining the pharynx to the 

 outside and formed by the closure of a groove in the side of the 

 pharynx. The significance of this organ is still obscure ; it is presum- 

 ably connected with the feeding process, which begins at this stage. 

 It has been thought to represent a gill-slit. 



Fig. 20. Larval amphioxus before metamorphosis. 



an. anus; c.s.g. club-shaped gland; end. endostyle; g. gill-slits lying on right 



side, which will later move over to the left. H.p. Hatschek's pit (left first 



coelomic sac); m. lower edge of mouth, lying on left side; n. notochord; 



p. pigment spot. 



The first two coelomic pouches differentiate, asymmetrically, at 

 this time. That on the right becomes the coelomic cavity of the head 

 region, while the left one acquires an opening to the exterior and a 

 heavily ciliated surface. This is perhaps also connected with the feed- 

 ing-systems and becomes developed into Hatschek's pit of the adult. 

 Its interest to the morphologist lies in the fact that the first coelomic 

 cavity opens to the exterior in other early chordates and in some 

 vertebrates (p. 206). The pit has thus some claim to be considered the 

 equivalent of the hypophyseal portion of the pituitary gland. 



Further gill-slits develop in the mid-ventral line and move over on 

 to the right side until fourteen have been so formed. Meanwhile, a 

 further row of eight slits appears above that already formed. These are 

 the definitive slits of the right side and presently the larva proceeds to 

 become symmetrical by movement of eight of the first row of slits 

 over to the left side, the remainder disappearing. At this 'critical 



