ectoderi"n. In the test they i'e[)i'oihice freely and secrete 

 lar^e quantities of the tuiiicine matrix. 



Many of the cells in the test remain small and simple, 

 as the rounded, fusiform, or stellate test cells (fif^. 5, t.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 mentida, form a 

 well-marked vesicular layer (PI. III., fig. 9, hi.). 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 



