42 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
ectoderm. In the test they reproduce freely and secrete 
large quantities of the tunicine matrix. 
Many of the cells in the test remain small and simple, 
as the rounded, fusiform, or stellate test cells (fig. 5, #.c.). 
Some become larger, much branched, elongate into fibres, 
or degenerate into globular pigment cells; others may 
store up reserve products; while others again are con- 
verted into the large vacuolated ‘‘bladder-cells”’ which, 
in the outer part of the test of Ascidia mentula, form a 
well-marked vesicular layer (Pl. IIL., fig. 9, 6/.). In this the 
structureless matrix contains innumerable closely packed 
spherical vacuoles, each with a thin peripheral film of 
protoplasm and a parietal nucleus. These bladder cells 
measure from 0°10 to 0°15 mm. in diameter. Some of 
them show more than one nucleus, and may be formed by 
the fusion of several cells. Some of them at least appear 
to be derived from the “‘ testa-cells’’ of the embryo, and 
are thus descendants of cells belonging to the follicle 
which surrounds the ovum. 
The test also becomes organised by the growth into it 
of the so-called ‘‘ vessels.’’ These are out-growths of the 
mesodermal body-wall, covered by ectoderm, and contain- 
ing prolongations of blood channels from the connective 
tissue of the body-wall. Plate II., fig. 5, shows such an 
out-growth, and exhibits the general relations of test 
(cuticle) ectoderm, and mesoderm. It also explains how 
it is that the blood channel being pushed out as a loop 
gives rise to the double or paired vessels seen branching 
through the test (Pl. III., fig. 9). The two vessels of a 
pair are one blood channel imperfectly divided by a con- 
nective tissue septum. The blood courses out along one 
side, round the communication in a terminal knob at the 
end and back down the other side. The terminal knobs 
are very numerous and form a marked feature in the outer 
