40 OUTLINES OF CHORDATE DEVELOPMENT 



if it be imagined that the formative centers located in the 

 most anterior region of the pharynx have been twisted out of 

 their normal positions, so that the morphological mid-ventral 



en 



gsl4 



o gsl 



m rm 



FIG. 14. Anterior ends of Amphioxus larvse. A. Larva with eleven primary 

 gill slits, from left side. After Goodrich. B. Larva with eleven primary gill 

 slits, from right side showing metapleural fold and secondary gill slit rudiments. 

 After Willey. C. Larva with fourteen primary gill slits, from left side. After 

 Lankester and Willey. ao. Ventral aorta (branchial artery); at, atrial cavity; 

 ch, notochord; en, cranial nerve; csg, club-shaped gland; dn, dorsal spinal nerve; 

 e, endostyle;0#, groove of Hatschek; gsl, gsll, gs!4, first, eleventh, and four- 

 teenth primary gill slits; Ilgs, rudiments of secondary gill slits; i, internal opening 

 of club-shaped gland; Im, lower margin of mouth; m, mouth; my, myotomes; 

 n, nerve cord; ne, nephridium; o, external opening of club-shaped gland; p, 

 pigment in nerve cord; r, renal cells in floor of atrial cavity; rm, edge of right 

 metapleural fold; ro, Raderorgan. 



line becomes displaced to the topographical right side, and 

 structures morphologically of the right and left sides appear, 

 respectively, dorsally and ventrally on the right side alone. 



