W. K. BROOKS ON THE GENUS SALPA. 23 



embryo and form the epithelial capsule are flat, as shown at B'. This 

 difference becomes more and more marked and the transition more and 

 more abrupt, until the lower thickened portion, cut B, 23, becomes sharply 

 separated from the epithelial capsule B', and forms what I shall call the 

 supporting ring of the placenta. This is shown at various stages of 

 development at 23, in Plate XVIII, Plate XXXV and Plate XLV, and in 

 cuts B, C and D. 



One of its functions, and apparently the only one in most species, is 

 to act as a framework for the placenta, and a support to hold the embryo 

 in its position above the placenta ; but it also has a nutritive function in 

 at least one species, Salpa pinnata, and its cells ultimately degenerate and 

 become converted into food for the embryo. 



The rupture which sets the fully grown embryo free usually occurs 

 around the neck of the placenta, so that the supporting ring is carried 

 away with it and is gradually absorbed. 



While the embryo projects into the cloaca of the chain-salpa as I 

 have shown, it is not at first in direct contact with the water, for it is 

 covered, in the first place, by the epithelial capsule, and in the second 

 place by the embryo sac, which is now to be described. This structure 

 is often called the amnion, as it bears a certain resemblance, in its 

 anatomical relations, to the amnion of the higher vertebrates, although it 

 is not formed, as it is in the vertebrates, from the tissues of the embryo, 

 but from those of the chain-salpa. It first makes its appearance as a cir- 

 cular ridge or fold, Plate XLV, Fig. 2, 21 and 22, in the wall of the cloaca, 

 around the area where the embryo is attached by the neck of the placenta. 

 In some species it seems to be absent ; in others, as in Salpa hexagona, it 

 is never any more developed than it is in the figure just referred to. 

 More usually, however, it grows up around the embryo until this is com- 

 pletely shut in except for a small pore or unclosed space. It is shown at 

 21 and 22 in transverse sections of Salpa pinnata in Plate XVIII, Figs. 

 1-6, in longitudinal section in Fig. 8, and in surface view in Plate XLI, 

 Fig. 1. The space between the embryo sac and the embryo is the brood 

 chamber. In its origin this is part of the cavity of the cloaca, but it 

 becomes completely shut off except for the pore, which is shown in Plate 

 XVIII, Fig. 4. The wall of the embryo sac is double, and the space 

 between its two folds is continuous with the blood-spaces of the chain- 

 salpa. 



It is plain from this description that a horizontal section, like the one 

 in Plate XIII, Fig. 3, in the plane of the line marked xm, 3 in cut, will 



