PYROSOMA FURTHER DEVELOPMENT OF THE BUDS. 491 



the contiguous walls of the entoderm-sac and the peribranchial sacs 

 (Fig. 258, p), but they very soon become elongate (Fig. 259) 

 The inner longitudinal bars of the branchial sac (Fig. 253, If) now 

 develop at right angles to the clefts. As the slits break through 

 chiefly in consequence of an outgrowth of the entoderm-sac, they 

 appear to be lined with entoderm. 



The two peribranchial sacs, which early lost their connection with 

 those of the adjacent bud, by growing towards each other on the lower 

 side of the stolon and fusing, give rise to the impaired atrial cavity 

 (Figs. 253, kl, 259, d). 



Finally, the outgrowths of the dorsal wall of the branchial sac 

 known as the languets (Fig. 253, rz) develop. 



The rudiment of the central nervous system appears at first as a tube 

 running along the whole length of the upper side of a stolon-segment 



*<*&*,' 



V_iL n \ eb 



e 



a 



E 



FIG. 258. Stolon of Pyrommw with the rudiments of two individuals, / and // (after 

 SEELIGER). A, side view; 7J, seen from the side on which the genital strand is 

 situated, eb, rudiment of elaeoblast ; ec, ectoderm ; es, endostyle-rudiment ; g, 

 genital strand ; ks, gill-clefts ; HI, rudiment of the stomach and intestine ; n, nervous 

 system of bud 1 ; n', nervous system of bud 11 ' ; p, peribranchial tubes ; s, rudiment 

 of the lateral nerves (in B, seen in transverse section). 



(Fig. 256, n) ; later, however, the proximal part of this tube develops 

 into a large vesicle, while the thinner, distal part disappears. Two 

 lateral outgrowths of the proximal part of the neural tube (Figs. 258, *, 

 259) can be seen very early (Fig. 257 A, sn) ; these soon change 

 into hollow processes which encircle the alimentary canal and unite 

 on its lower side. These are the rudiments of the so-called lateral 

 nerves. A growth of the cells on the dorsal wall of the cerebral 

 vesicle yields the rudiment of the ganglion proper, which later gives 

 off the lateral nerves that run backward along the dorsal surface of 

 the adult in the form of solid strands with terminal ganglionic 

 swellings. The remains of the cerebral vesicle develop a connection 

 with the branchial sac and become transformed into the ciliated pit 



