LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 213 



of development while still confined to the lower portion, they 

 then grow into the upper portion also. 



Thus the oral hood itself has a double origin ; it may be 

 said to arise from two independent ebauches (to employ a 

 French term which has not, when used in this connection, a 

 satisfactory English equivalent, '' forecast" being the nearest 

 approach), while the system of the buccal cirri arises from a 

 single ebauche. 



The formation of the cartilaginoid elements which form the 

 skeletal or supporting tissue of the buccal cirri, and which, 

 taken together, may be referred to as the buccal skeleton, 

 commences in Stage I. In this stage we find first one and 

 then two minute circular and homogeneous differentiations in 

 the mesoblast of the lower lip of the mouth, which can be seen 

 through the outer integument. They appear some distance 

 behind the external orifice of the club-shaped gland, just below 

 the mouth. They commence to form before the mouth has 

 begun to turn round in front. 



In the succeeding stages fresh elements are added at both 

 ends — anterior and posterior — so that the central elements are 

 always the oldest ; and they soon begin to get irregular in out- 

 line, indicating a tendency to send out processes. Eventually 

 each piece gives rise to one process. 



As the differentiation of the elements of the future buccal 

 skeleton proceeds the superjacent integument becomes raised 

 up into a fold, which is the lower or right fold of the oral hood. 

 About Stage IV the above-mentioned processes, which grow 

 out from the separate elements, attain such a size that the in- 

 tegumentary fold over them is thrown into a series of pleats 

 (fig. 6, &c.). By an extension of this pleating the buccal cirri 

 of the adult are formed. 



From Stage V onwards the lower fold of the oral hood meets 

 the upper fold, and the point of junction is very plainly marked 

 by a ridge, under which the buccal skeleton penetrates into 

 the upper fold (figs. 8, 12, 13, and 16). 



The lower fold is continued anteriorly some distance bevond 

 the buccal skeleton as a ridge. Eventually the cirri extend 



