II STRUCTURE—THE TEST 41 
ege-follicle cells, see below, p. 56), which have passed through 
the ectoderm. This process commences in the larval state with 
the migration of mesenchyme cells from the blastocoele through 
the epiblast. Ectoderm cells, and possibly also some primitive 
endoderm cells, also take part in forming the test. Many of 
these cells in the test remain small and simple, as the fusiform 
and stellate test-cells; some become pigment-cells, while others 
enlarge and become vacuolated to form the large (up to 0°15 
mim. in diameter) vesicular or “ bladder” cells—this is especially 
the case in the outer layer of the test in Ascidia mentula (see 
TiESin MANTLE 
O Ke "6 re Q o£ 
Cha 2D: Oo te 
PAE" 200 
5 
ri 
\ 
bLc. Ev. LE ec. MEsc. GI's. 
Fic. 16.— Diagrammatic section through test and mantle of Ascidia to show the rela- 
tions of ectoderm to body-wall and cuticle. 6l.c, Bladder-cells ; 6/.s, blood-sinus ; 
c.t.c, connective tissue cells ; ec, ectoderm ; mes.c, wandering mesoblast cells ; m./, 
muscle fibres ; ¢.c, test-cells ; ¢.v, “vessel” of the test.” 
Fig. 17, 61) where there are innumerable clear vesicles, each sur- 
rounded by a thin film of protoplasm and having the nucleus 
still visible at one point of the surface. In some of the Tunicata 
the test-cells produce calcareous spicules of various shapes (see 
below, p. 86). 
The test also becomes organised by the growth into it of the 
so-called “ vessels.” These are outgrowths of the body-wall 
covered by ectoderm and containing prolongations of blood- 
channels from the connective tissue of the “mantle” (body-wall). 
Fig. 16, tv shows such an outgrowth, 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 
