20 MORPHOLOGY OP 



begun (fig. 45). The walls of the branchial region are 

 distinctly differentiated from those of the rest of the gut, 

 consisting of long, solid-looking cells arranged in layers of one 

 to two deep. The lumen of the gut is very small anteriorly 

 and has an irregular outline in preserved specimens (figs. 36 

 and 37), but in the middle of the collar region as at present 

 marked out, its lumen is continuous dorsally with that of the 

 notochord (fig. 39). 



This notochord, which is now a very prominent feature in 

 sections of the anterior end of the body, arose in the first 

 instance, as already stated, by a forward growth of the anterior 

 dorsal wall of the pharynx, which thus shuts off a short diver- 

 ticulum of hypoblast (fig. 30). The part of the pharynx with 

 which the walls of this diverticulum are continuous, then 

 separates itself from the rest by longitudinal constriction, which 

 at first causes the lumen of the gut to take an 8-shaped figure, 

 the separation of the dorsal part of the 8 becoming finally 

 complete from before backwards. This process gives rise to the 

 appearance seen in fig. 37. The part thus constricted off 

 becomes then entirely separated except at its posterior end, 

 where throughout life its lumen opens into the pharynx. 



In its anterior region the lumen of the notochord is always 

 suppressed at this stage, owing to the compression of the ventral 

 against the dorsal wall. Moreover, in larvae of this, as of all 

 subsequent stages, the lumen is altogether obliterated in part 

 of its course. This obliteration does not appear to occur pro- 

 gressively from before backwards, but more or less irregularly, 

 so that, as in fig. 38, the lumen may have already disappeared 

 while still present in a region anterior to this (fig. 36). As, 

 however, in older animals the lumen is always continued far 

 into the notochord of the proboscis cavity (namely, to a point 

 anterior to that where it is already obliterated in two-gill 

 larvae), it is almost certain that the subsequent increase in the 

 length of the notochord is due to a growth from behind for- 

 wards, and that all the notochord which is as yet formed 

 (two gill-slits) is pushed bodily forwards by a proliferation, 

 probably occurring at the point of union with the gut. The 



