578 



DEVELOPMENT OF THE LANCELET 



case of the tadpole (p. 202), except that the central 

 canal of the medullary cord (//) is formed after the 

 plate has become separated from the outer ectoderm. 

 In the mid-dorsal line a thickening of the endoderm 

 (cJi) soon becomes constricted off to form the notochord 



FIG. 154. Embryo of Amphioxvs. 



A, in vertical section, slightly to the left of the middle line; B, in horizontal 

 section, ak. ectoderm ; en. neurenteric canal ; dk, ud. archenteron ; ik. endo- 

 derm ; ink. mesodermal folds ; n. medullary canal ; us. first ccelomic pouch ; 

 nsh. coelomic cavity ; V. anterior ; H ' . posterior end. (From Korschelt and 

 Heider's Embryology, after Hatschek.) 



(pp. 203, 419 and 441), and on either side of this 

 a series of hollow endodermic pouches arise, arranged 

 metamerically (Figs. 153, mk, and 154, us, mfc). The 

 cavities of these, which subsequently give rise to the body 

 cavity ccelome (Ih, usJi), are thus at first in free communica- 

 tion with the archenteron and are known as ehteroc&les ; 



