Zool— Vol. I.] BANCROFT— CHELYOSOMA PRODUCTUM. 315 



prominent fibres are seen running at right angles to the 

 surface. 



The outer layer, which corresponds to the yellow external 

 layer of Lowig and Kolliker (1846), Schulze (1862), and 

 Hertwig (1872), is, as they state, different from cellulose. 

 Towards reagents it behaves like the substance examined 

 by Schulze, giving no blue color with Potassium Iodide 

 and Sulphuric Acid, and being insoluble in concentrated 

 Sulphuric Acid. Boiling with dilute alkali does not 

 dissolve it, but hot concentrated Potassium Hydrate dis- 

 solves or at any rate disintegrates the layer. It is slightly 

 harder, stiffer, and more brittle than the tunicin layer, but 

 is formed from it as the sequel will show. In sections it 

 takes a deeper stain than the cellulose, is much more 

 highly refractile, and usually appears homogeneous, but 

 sometimes is differentiated into layers (fig. 5). In some 

 places, however, where it does not seem to be fully formed, 

 it has a more granular appearance. It covers the whole 

 surface of the ascidian, being thinnest on the hinges, a 

 little thicker than the average on the disk, and thickest at 

 the edges of the orifices. It does not cover the surface of 

 the test that extends into the siphons. In addition to 

 covering the surface it is also found within the test in the 

 region of the siphonal ridges which project inwards from 

 the median line of each siphonal plate; but here it occurs 

 in its less compact granular form. As the most powerful 

 muscles of the body are attached to these ridges they must 

 be quite strong; and it seems that the extra supply of yel- 

 low substance is introduced to add rigidity to the structure. 

 On the contrary, where flexibility is required, as in the 

 hinge, the thickness of the yellow surface layer is reduced 

 to a minimum and additional fibres added to the tunicin 

 layer. 



Cells in the Test. 



All the functionally active cells in the test appear to be 

 of mesodermic origin. In rare cases I have found cells 

 that have been derived from the ectoderm. They have 



