REVIEW OF SPENGEL’S MONOGRAPH ON BALANOGLOSSUS. 393 
the proboscis stalk, however, also gives rise to skeletal matter, 
which behind fuses with the notochordal excretion, but in front 
is separated from it by a wedge of cells, the solid tips of the 
ventral pockets of the proboscis cavity. This ectodermal rod is 
called by Spengel “the tooth of the skeleton” (PI. 30, fig. 9 7.). 
The notochordal skeleton is thinnest in front, where it supports 
the ventral wall of the ventral sac of the notochord, and thickest 
under the neck portion. Where the neck passes into the 
buccal cavity the skeletal rod splits into two rods, which pass 
to the right and left of the buccal cavity embedded in slight 
lateral grooves in the wall of the same, to whose cells they owe 
their existence. Now Bateson maintained that the hinder 
part of the notochord was pinched off from the gut-wall by 
lateral grooves, and Spengel has been able to supply a most 
interesting confirmation of thisidea. If we cut sections of the 
hinder part of the undivided skeletal rod, we find that whereas 
the younger layers are arranged concentrically round a single 
centre, the older layers form two systems of concentric curves 
round two separate centres to the right and left respectively 
of the new centre. Hence where now the skeleton is a single 
rod secreted by the ventral wall of the notochord, it used to 
be in the form of two rods lying in grooves of the lateral walls 
of the buccal cavity, but these grooves have coalesced and 
separated off the notochord. Pl. 30, figs. 8—13, show the 
form of the collar skeleton in several of the species. 
In addition to this primary skeleton there is developed in 
all species, though much more in some than others, a supple- 
mental skeleton, the chondroid tissue. It is the special 
merit of Marion to have first called attention to this tissue. 
It consists of a structureless ground substance with nume- 
rous groups and strings of cells embedded in it, presenting in 
transverse section a strong resemblance to cartilage. In lon- 
gitudinal section, however, it is seen that these cellular masses 
are outgrowths from the walls of the collar coelom, and in some 
cases also from the wall of the dorsal pockets of the proboscis 
cavity (i.e. the portion of the proboscis cavity which ends in 
the pore and the corresponding blind portion on the right). 
