84 AMPHIOXUS. 



diameter along its whole length. Later on, it enlarges very 

 greatly, and, pushing the ventral body-wall before it, en- 

 croaches on the space hitherto occupied by the ccelom, finally 

 extending so far dorsal wards as nearly to surround the ali- 

 mentary canal (Fig. 41, AC; cf. also Figs. 12 and 13). 



The primary gill-slits at first open directly to the exterior, 

 but, as they lie between the two metapleural folds, they become 

 boxed in on the formation of the floor of the atrial tube, and 

 from this time open into the atrial tube or chamber. The 

 secondary gill- slits, which also lie between the two metapleural 

 folds, very close to the base of the right metapleural fold, are 

 not formed until the floor of the atrial chamber is completed, 

 and consequently open into this chamber from the first. 



The metapleural folds are at first solid ridges ; large spaces 

 soon appear in them, which become the metapleural canals of 

 the adult (Figs. 12, 13, and 41, MV). 



b. The Mesoblastic Somites. 



At the commencement of the larval period fourteen or fifteen 

 pairs of somites are present ; during the early part of this 

 period the number steadily increases, and, shortly before the 

 appearance of the secondary gill-slits, the full number of somites 

 of the adult animal, which appears to be very generally sixty- 

 one, is attained. The somites formed during the larval period 

 differ from those developed in the embryonic stages in not com- 

 municating with the mesenteron at any time in their formation. 

 In the development of these hinder somites it is probable that 

 the polar mesoblast cells take an important share. 



Concerning the further development of the somites some 

 interesting details are given by Hatschek. At the commence- 

 ment of the larval period, i.e. about the time of formation of 

 the mouth, each somite (cf. Figs. 32 and 42) becomes divided 

 into a dorsal portion or proto vertebra, and a ventral portion or 

 lateral plate. 



The proto vertebras retain the original segmental arrange- 

 ment, i.e. the cavities of successive protovertebras remain 

 separate from one another ; but in the ventral portions of the 

 somites, or lateral plates, the septa become absorbed, and the 

 cavities open into one another along the whole length of the 

 body, forming a continuous body cavity or ccelom. 



