48 JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGRAPHS. 



longitudinal section in Plate XXXV with the transverse sections in Plate 

 XVIII, and the horizontal sections in Plate XLVI, Figs. 2, 3 and 4. 

 In a horizontal section near the top, Fig. 2, the cavity is cut up by strings 

 and clumps of cells which have no constant arrangement, although the 

 study of sections at a lower level shows that certain channels, bl, 1 and 

 bl, 2, communicate with one or the other of the two chambers which 

 open into the blood system of the chain-salpa, while others are channels 

 of communication between one of these chambers and the other. At a 

 somewhat lower level, Fig. 3, the strings of cells gradually unite to form 

 a transverse partition which separates the anterior chamber, bl, 7, from 

 the posterior, bl, 2, and these chambers gradually become more and more 

 capacious, and the blood channels in the partition less and less numerous, 

 until, near the neck of the placenta, Fig. 4, the partition becomes, in 

 longitudinal sections, a straight rod, running across the placenta like the 

 handle of a basket. 



In a section just above the neck of the placenta the ends of this rod 

 are separated from the supporting ring by a space, 33, which is also 

 shown at 33 in Plate XXXV. This space runs around the whole circum- 

 ference of the placenta, as shown in Fig. 4, and it is bounded internally 

 by the endothelium of the placenta and externally by the supporting ring. 

 It is interrupted at short intervals by fibers which cross it, and it is 

 lined by very small cells, very much smaller than the blood corpuscles. 

 This space acts, perhaps, as a valve to close or expand the opening of the 

 placenta, although sections of preserved specimens throw little light on 

 its function. The strings of cells are nourished by the plasma of the 

 blood which is retarded in the meshes of the spongy mesh work, and also 

 by the blood corpuscles; and in both Salpa pinnata and Salpa hexagona, 

 blood corpuscles may be found settling upon the surfaces of the strings 

 and sinking into their substance, as is shown in Fig. 5 of Plate XLVI, 

 which is a highly magnified drawing of part of Fig. 3. 



SECTION 8. TJie Nutrition of the Embryo. 



As the mammalian placenta nourishes and aerates the blood of the 

 foetus by the diffusion of gases and food in solution through the walls 

 of the blood-vessels, it has been generally taken for granted that the 

 placenta of salpa performs its function in the same way, and it has 

 been described as divided into a foetal chamber and a maternal chamber, 

 although its cavity is in reality part of the body cavity of the chain-salpa, 

 and the blood which circulates in it that of the chain-salpa. The salpa 



