356 



TUNICATA. 



layer. It has hitherto been believed that, as 0. HEBTWIG (No. 25) 

 maintained, the cells that wandered into the cellulose substance came 

 from the ectoderm, but KOWALEVSKY (No. 32) has recently proved* 

 that the mantle-cells were derived from the mesoderm, being meso- 

 dermal cells which traverse the ectoderm and thus migrate outwards 

 (Fig. 166). Subsequently, in the cellulose substance, they assume 



a ^ 



...ot 



(UL 



FIG. 167. Embryos of Phallusia mammillata at a later stage (after KOWALEVSKY)' 

 A, lateral aspect ; B, dorsal aspect, au, eye ; ch, chorda ; cl, cloacal vesicle ; d, 

 rudiment of the alimentary canal ; en, entoderm-sac ; /, ciliated pit ; h, adhesive 

 papillae ; i, mouth ; vis, mesenchyme-cells ; ot, otolith ; r, trunk-section, s, caudal 

 section of the medullary tube ; sb, sensory vesicle ; sc, sub-chordal entoderm- 

 strand ; v, vacuoles between the cells of the chorda. 



the character of star-shaped connective -tissue cells. We have to 

 regard the mantle of the Tunicates as a cuticular gelatinous secretion 

 permeated by phagocytes (mesoderm-cells).t When, in the composite 

 Ascidians, some of the individuals disintegrate, the phagocytes of the 



* SALENSKY'S recent statements as to Pyrosoma (No. 74) fully agree with this 

 observation (see below, p. 401). 



[f SEELIGER (No. XXXIII.) agrees with KOWALEVSKY that the true test- 

 cells are mesodermal. In Oikopleura, however, the cells of the "Haus" are 

 ectodermal. ED.] 



