100 WILLIAM BATESON. 
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 larve 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 
larve), 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 
