84 AMPHIOXtrS. 



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 coelom, finally 

 extending so far dorsalwards 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 metaplenral 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 metaplenral 

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

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

 and consequently open into this chamber from the first. 



The metaplenral folds are at first solid ridges ; large spaces 

 soon appear in them, which become the metaplenral canals of 

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



8. The Mesoblastic Somites. 



At the commencement of the larval period fourteen or fifteen 

 f)airs of somites are present ; during the early part of this 

 period the number steadily increases, and ^ shortly before t he 

 appearanc e of the secondary gill-slits, the full number of somit es 

 o f the ad ult animal, which appears to be very generally sixty- 

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

 diflTer 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 mes oblast cells t a ke an important shar e. 



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 som ite (cf. Figs. 32 and 42) becomes divided 

 i nto a dorsal portion or protovertebr a, and a ventral portio n or 

 l ateral plate. 



The protovertebree retain the oriffl nal segmental arrange- 

 m ent, i.e. the cavities of successive ' protovertebrte 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 coelom. 



