LATEE LARVAL DEVELOPMENT OF AMPHIOXUS. 199 



independent of one another in their origin^ the elements of the 

 buccal skeleton arise entirely unilaterally from a single differen- 

 tiation of mesoblastj which grows at both ends, and is situated 

 primarily in the right or lower half alone of the oral hood, and 

 secondarily continues itself into the left or upper half. 



The lower edge of the oral hood is prolonged anteriorly as a 

 ridge, into which the buccal skeleton is subsequently con- 

 tinued. 



With regard to the variations met with in this stage, it is 

 only necessary to say that the number of secondary slits varies 

 from seven to nine ; and as to the primary slits, it is usual to 

 find the twelfth, sometimes also the thirteenth, on the point of 

 closing ; but I have found an instance in which, although there 

 were nine secondary slits and the club-shaped gland was 

 beginning to atrophy, yet there were fifteen primary slits, all 

 in a healthy condition, with the exception of the first, which 

 was closing. 



Stage VI.— Figs. 11, 12, and 19. 



The constant characteristic for this stage is that the primary 

 and secondary gill-slits are of equal size and similar shape for 

 the most part ; and, excluding the first two and the last two on 

 each side, they are approximately as broad as they are long. 

 Further, the tongue-bars in most of them have fused with the 

 ventral borders of their respective slits. 



The first primary slit has entirely gone, and it is usual to 

 find the eleventh primary slit, together with the twelfth and 

 sometimes also the thirteenth (figs. 11 and 12), in a state 

 bordering on closure. All the slits of the left side are now, as 

 a rule, entirely confined to that side throughout their whole 

 extent (cf. fig. 8), with the exception of the ninth, which 

 bends under ventrally. The first slit of the left side in this 

 and all succeeding stages is the original second primary slit ; 

 and similarly the ninth slit of the left side is the original tenth 

 primary, and so on for the other primary slits. 



Owing to the atrophy of the first primary slit, the first slit 

 of the right side — i. e. the first secondary slit — comes to lie 



