STRUCTURE THE TEST 



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 



TEST 



MANTLE. 



FIG. 16. Diagrammatic section through test and mantle of Ascidia to show the rela- 

 tions 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." 



Fig. 17, U~) 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, t.v 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 



