Zool.-Vol. I.] BANCROFT— CHELYOSOMA PRODUCTUM. 317 



instead of oval as in the other parts. If, however, the 

 matrix is homogeneous, the bladder cells nearest to the ecto- 

 derm are just as large as those in the center and differ from 

 them only in having in some cases a slightly larger amount 

 of cytoplasm. We see then that as these cells come to lie 

 in the outer layers of the test the cytoplasmic and nuclear 

 material diminishes and the test cavity remains the same. 



Origin of the Yellow Layer. 



It has been said that the outer yellow layer of the test is 

 formed by the activity of the cells in the tunicin layer. The 

 steps in the process can be followed quite easily, but unfor- 

 tunately the principal criterion that must be used in follow- 

 ing out the stages cannot be well represented in the 

 drawings. This is the characteristic appearance of the 

 yellow substance due to its homogeneous structure and 

 high index of refraction. In some regions, especially in 

 the siphonal ridges, it is found in a granular, more lightly 

 colored form, thus losing some of its characteristic appear- 

 ance ; but it retains its high refractility and grades insensi- 

 bly into the more compact form so that there can be no 

 doubt of its identity. The first step in the formation of the 

 yellow substance is the deposition of a thin layer of it in 

 the vicinity of the wall of a bladder cell. The deposit may 

 be of the compact or granular form, and either just within 

 or without the wall of the bladder. Figure 9, a, shows a 

 cell with a thin layer of the substance just inside the 

 bladder, e, a cell with the substance deposited on the out- 

 side only. Figure 9, b, represents a stage in which the 

 deposit has been principally within, as shown by the angu- 

 lar vesicle. The size of the deposit about many of the 

 cells, such as c, fig. 9, and a, b and c, fig. 5, shows that 

 most of it has been laid down on the outside of the bladder. 

 Appearances like a and b, fig. 5, where isolated masses 

 surrounding individual cells fuse with the outer layer, are 

 very numerous and the fusion is always perfect, no line of 

 contact being visible at all. This together with the identical 



