42 OUTLINES OF CHORDATE DEVELOPMENT 



superficial layer of transparent ectoderm cells. The tongue 

 bar grows down like a stalactite from the roof of the gill pouch, 

 and just before it reaches the ventral side of the pouch the 

 ectoderm is perforated opposite the tongue bar. But by this 

 time the atrial cavity has formed in this region, as will be 

 described later, so that the gill slits never open directly to the 



cv 



dgsi 



FIG. 15. Anterior ends of Amphioxus larvae showing the migration of the 

 mouth, formation of tongue bars, reduction in primary gill slits, etc. After 

 Willey. A. Larva with fourteen primary gill slits nearly in their definitive posi- 

 tion on the left side. Oral hood in process of formation. B. Larva with nine 

 primary, and eight secondary gill slits. Mouth nearly in its definitive anterior 

 position and oral hood nearly completed. This larva has nearly reached the 

 "critical stage." at, Atrial cavity; 6, buccal (oral) cirri; be, buccal cavity (cavity 

 of oral hood); c, coelom; cv, cerebral vesicle; df, dorsal fin chambers; dgsl, dgs8, 

 first and eighth definitive gill slits (second and ninth of the primary series) ; dph, 

 dorsal wall of pharynx (region of epipharyngeal groove); e, endostyle; gsl, gs9, 

 gsl4, first, ninth, and fourteenth primary gill slits; h, hepatic caecum; i, intestine; 

 Im, lower margin of left metapleural fold; ra, mouth; nH, Hatschek's nephridium; 

 pb, peripharyngeal band; r, renal papilla in floor of atrial cavity; ro, Raderorgan. 

 tb, tongue bar; uh, upper margin of oral hood; v, velum. 



outside; the more anterior primary gill slits apparently form 

 exceptions to the usual arrangement in opening directly for a 

 time (Fig. 17). As the external openings of the pouches en- 

 large, the tongue bars move to the surface and if it is true 

 that they are completely covered with endoderm, the atrial 



