142 



DEVELOPMENT OF AMPHWXUS. 



from the latter, towards the completion of the rotatory- 

 growth, which carries the slits from one side to the other, 

 that the tongue-bars arise (Fig. 80). 



The vertical and longitudinal axes of most of the slits, 

 both primary and secondary, are now almost equal , but 

 the original difference in this respect, which we noted 

 above, is still to be observed in the case of the foremost 

 and hindmost slits of the two series. (Cf. Fig. 80, s.s^ 

 and p.s'-, and i-..s-s and p.s^^.) The first primary slit has 



Fig. 80. — Anterior portion of larva of same age as in Fig. 79, seen from the 

 ventral surface. The pharynx is flattened out. (After WiLLEY.) 



ch. Notochord. m. Entrance to mouth, v. Velum, p.s^. Vestige of first 

 primary slit. pS-. Secondary primary sht. /.ji". Tenth primary slit. /.ji-. Ves- 

 tige of twelfth primary slit. j.ji. First secondary slit. e. Endostyle. i.s. Eighth 

 secondary slit. a. Atrium, pressed aside. 



now completely closed up, and its former existence is 

 barely indicated by a loose granular appearance at the 

 place it formerly occupied. 



The alternation of the gill-slits of the two series comes 

 out very clearly in Fig. 80. In most of the secondary 

 slits the formation of the tongue-bars is completed ; but 

 not so in any of the primary slits, where it is only be- 

 ginning. 



There are now eight secondary slits, an additional one 

 having been added behind, alternating with the ninth and 

 tenth primary slits. Usually the formation of secondary 

 slits stops at this point, no more being formed until the 



