W. K. BROOKS ON THE GENUS SALPA. 47 



transverse section through the anterior portion, Fig. 4 through the 

 middle portion, and Fig. 2 through the posterior portion of the body of 

 the placenta. The cellular strings become more and more numerous and 

 crowded as the placenta grows, as is shown by the series of stages, but 

 they are always most developed in the middle plane, as is shown in Fig. 

 4, while in front and behind the spaces are much larger, and the cellular 

 strings more independent. At the stage shown in Fig. 1 the blood 

 circulates vaguely back and forth into and out of the placenta, but hori- 

 zontal sections show that at the stages of Figs. 2, 3 and 4 the neck of the 

 placenta is divided into two openings, an anterior and a posterior, by a 

 median transverse partition. This partition is not flat, but folded in 

 such a way that no single vertical section can show the whole of it, but 

 each series shows it at all positions between that of Fig. 2, where it is 

 united below the neck of the placenta to the wall of the cloaca on the 

 left, to Fig. 3 where it joins it on the right. 



The partition is continuous with the " blood-bud," 24, and is formed 

 from a substance which appears to be the same as the cellulose mantle. 

 This gelatinous substance is formed in many parts of the body of salpa, 

 between the mesodermic endothelium of the body cavity and the endo- 

 dermal structures. 



It is shown, for example, at 32 in Figs. 3 and 5 of Plate XXXIV. 

 This partition divides the cavity of the placenta into an anterior blood 

 chamber, Plate XLV, Fig. 3, bl, 1, and a posterior chamber, Fig. 2, bl, 2, 

 which communicate with each other through the spongy mass formed 

 by the plexus of strings of cells which fills the middle portion, as shown 

 in Fig. 4. 



In Salpa pinnata the partition is formed by the "blood-bud," 23, 

 itself, which in advanced embryos fits like a plug into the neck of the 

 placenta in transverse section, Plate XVIII, Fig. 4, while in longitudinal 

 section, Fig. 8, or in a transverse section in front of the plug, Fig. 2, or 

 behind it, Fig. 5, there is a free channel for the entrance or exit of blood, 

 as is shown also in Plate XXXV, and in the series of horizontal sections 

 from the same stage shown in Plate XLVI, Figs. 2, 3 and 4. The blood 

 which enters the placenta behind this partition and passes into the 

 chamber marked bl, 2, must make its way slowly through the spongy 

 mass of cells before it can gain access to the anterior chamber, bl, 1, and 

 escape from the placenta, and the conditions are of course essentially the 

 same when the circulation is reversed. The general anatomical structure 

 of the placenta of Salpa pinnata can be understood by comparing the 



