THE TEST.—The test is notable amongst animal structures for containing "tunicine," a substance which appears to be identical in composition, and in behaviour under treatment with various reagents, with cellulose. It is cartilaginous in appearance and consistency, and to some extent in structure, as it consists of a clear (or in some cases fibrillated) matrix in which are embedded many corpuscles or cells. It is the matrix that contains the cellulose, which may form over sixty per cent by weight of the entire test. As the test is morphologically a cuticle, being a secretion on the outer surface of the ectoderm (Fig. 16, _ec_), the cells it contains have immigrated to it from the body, and it has recently been shown that many of these are mesodermal cells (leucocytes or connective tissue wandering cells, amoebocytes, and in some cases embryonic "kalymmocytes," or {41}egg-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 mm. in diameter) vesicular or "bladder" cells—this is especially the case in the outer layer of the test in _Ascidia mentula_ (see Fig. 17, _bl_) where there are innumerable clear vesicles, each surrounded 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. p. 86). [Illustration: FIG. 16.—Diagrammatic section through test and mantle of _Ascidia_ to show the relations of ectoderm to body-wall and cuticle. _bl.c_, Bladder-cells; _bl.s_, blood-sinus; _c.t.c_, connective tissue cells; _ec_, ectoderm; _mes.c_, wandering mesoblast cells; _m.f_, muscle fibres; _t.c_, test-cells; _t.v_, "vessel" of the test."]