392 E. W. MACBRIDE, 
“ Grenzmembran,” implying that it is only found where two 
tissues meet ; but it seems to me that he unnecessarily obscures 
its nature by this mode of expression: since he allows that it 
is in this “ border membrane” that the blood-vessels are per- 
forated, it follows that the excretions of two epithelia are 
sometimes separated by a remnant of the blastocele. This 
cuticular substance is specially thickened (being deposited in 
the form of concentric layers) to form the skeletal rod under- 
lying the notochord, and the skeleton supporting the U-shaped 
gill-slits. The latter, as being the simpler, will be explained 
first. Adjacent gill-sacs are apposed along their inner borders. 
This region is called by Spengel a gill septum. The apposed 
epithelia give rise to a septal bar; this shows, however, its 
double nature by a longitudinal furrow, and by diverging below 
into two. The “tongue,” as we have seen, contains a portion 
of the ceelom, and hence it gives rise to two bars, which owe: 
their existence mainly to the epithelium of the gill-sac, though 
the ccelomic mesoblast contributes also to a certain extent. 
Each septal bar is connected by adorsal arch with the posterior 
bar of the tongue in front, and the anterior one of the tongue 
behind, so that the entire gill-skeleton consists of a number of 
three-pronged bars, as shown in PI. 29, fig. 13a. 
To understand the relations of the skeletal rod underlying 
the notochord we must glance for a moment at the structure 
of the notochord itself as shown in fig. 2. We can distinguish 
in it a neck and a head portion. The former opens into the 
buccal cavity behind, and has a comparatively wide lumen 
throughout. The latter is almost solid (Bateson stated that 
it was quite solid, but Spengel maintains the existence of a 
narrow lumen), and runs out under the heart, pushing the 
epithelium of the proboscis ccelom before it, thus giving rise to 
two ventral pockets of this cavity (Pr. v. m.) separated by a 
septum. Behind, where the head joins the neck, it gives rise to 
a ventral sac with a widish lumen. Now the main portion of 
the skeletal rod is a cuticular excretion of the cells of the 
ventral side of the neck and the posterior aspect of the ventral 
diverticulum of the head ; the ectoderm of the ventral side of 
