46 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



SECTION 7. The Placenta. 



We have now to trace the history of the placenta, and as this organ 

 is much simpler in Salpa hexagona than it is in Salpa pinnata, I shall 

 first describe the origin and fate of the placenta of Salpa hexagona. 



The part of the follicle which is bathed by the blood of the chain- 

 salpa begins to grow, Plate XI, Fig. 3, as soon as the rest of the surface 

 of the embryo is covered by the epithelial capsule, and it soon becomes 

 many cells thick, and irregular folds and spaces appear in it. As soon as 

 the walls of the placental chamber, Plate XLV, Fig. 1, are formed by the 

 supporting ring, 23, the thickened portion of the follicle grows down into 

 it as a ring, 31, around its sides, and as a free pendant mass, 24, which 

 Barrois has compared to a bell-clapper. As the embryo grows, and the 

 cavity of the placenta becomes larger, Plate XLV, Figs. 2, 3 and 4, cell 

 multiplication goes on very rapidly in these structures, until the chamber 

 is completely filled by convoluted inosculating strings of cells, so arranged 

 as to break up the blood space into a number of tortuous channels, in 

 which the circulation is retarded, so that the plasma and the blood cor- 

 puscles are almost brought to rest, as they find their way through these 

 obstructed passages. Figs. 2, 3 and 4 of Plate XLV are sections through 

 the placenta and embryo of Salpa hexagona at three successive stages of 

 development, but they will also serve to illustrate the structure of the 

 placenta, since they cut it in three different planes. Figure 4 is from an 

 embryo at the stage of Plate III, Fig. 4, passing, through the neck of the 

 placenta and through the gill, o. Comparison with Plate III, Fig. 4, will 

 show that it is a vertical transverse section in the vertical axis of the 

 placenta. Figure 3 of Plate XLV passes through the neck of the 

 placenta and through the ganglion, s, and while it is from an embryo con- 

 siderably younger than Fig. 4 of Plate III, comparison with this figure 

 will show that it must cut the greater part of the placenta in front of the 

 median plane of Fig. 4. Fig. 2 is from a still younger embryo, passing 

 through the neck of the placenta, and through the place of the cloacal 

 aperture, g\ At this stage this point is very much farther forward than 

 it is in the embryo shown in the figure on Plate III, and nearly where 

 it is shown at g v in the embryo of Salpa pinnata shown in Fig. 3 of Plate 

 XLI, but examination will show that the section must cut the greater part 

 of the mass of the placenta about as much behind the plane of Fig. 4 as 

 the section in Fig. 3 is in front of it. While all these sections pass through 

 the neck of the placenta, the upper part of Fig. 3 is in effect a vertical 



