THE DEVELOPMENT OF AMPHIOXUS 



35 



C. THE FIRST GILL SLIT 



At the same time the mouth is forming, the endoderm 

 pushes out ventrally, opposite the second somite, and fuses 

 with another thickened patch of ectoderm (Fig. 9, C). Per- 

 foration follows and forms the first gill slit which rapidly enlarges 

 and moves up on the right side of the head, nearly opposite 



P np 



A 



ro 



ch 



gsi 



csg 

 P b Hn n P 



B 



gsi 



csg 



FIG. 12. Anterior ends of Amphioxus larvae, in optical section. A. One gill 

 slit stage, from right side. After Hatschek. B. Four gill slit stage, from left 

 side. After Lankester and Willey (mouth added). 6, Brain region; c, ccelom 

 (preoral head cavity); ch, notochord; csg, club-shaped gland; d, dorsal portion 

 of preoral pit (later forming the groove of Hatschek and Hatschek's nephridium) ; 

 e, rudiment of endostyle; gsl, first gill slit; Hn, Hatschek's nephridium; i, 

 intestine; m, mouth; n, nerve cord; np, neuropore; o, external opening of club- 

 shaped gland; p, pigment; ro, Raderorgan; v, cerebral vesicle. 



the mouth (Fig. 12, A}. It remains smaller than the mouth 

 but is similarly bordered with long cilia. 



D. THE ANUS 



Very shortly after the mouth and first gill slit are formed the 

 wall of the narrow enteron, immediately below the neurenteric 

 canal, fuses with the ectoderm and the region is perforated as 

 the anus. At first this lies in the mid-line but later is displaced 

 toward the left, by the development of the provisional caudal 



