W. K. BROOKS ON THE GENUS SALPA. 279 



Thus Leuckart (1) says that its physiology is like that of the mam- 

 malian placenta, and that, in origin, it is, as it were, the remnant of the 

 yolk left over as a yolk-sac after the embryo is formed, and that instead 

 of being turned to use directly in the construction of the embryo, it is 

 converted into a necessary organ of the foetus. Its modifications have 

 for their purpose simply to increase the surface by which the body of 

 the embryo comes into contact with the current of maternal blood, and 

 while it is at first solid, it soon becomes hollowed out into a cup-shaped 

 cavity which communicates with the circulatory organs of the mother, 

 and which is to be regarded as a maternal blood channel. Its inner 

 wall, which is freely bathed by the blood of the mother, exhibits 

 numerous irregular processes which, as a rule, run like ribs from the 

 top of the placenta to its opening. Often there is a knob-like projection 

 which hangs down from the roof of the cup to some distance into its 

 cavity. 



It will be seen that this brief sketch of its structure is accurate, and 

 while Barrois and Salensky have shown that it is no way comparable to 

 a yolk-sac, and that its chamber is not hollowed out of a solid rudiment, 

 their more elaborate accounts of its structure are less correct than 

 Leuckart's. 



Barrois (4) says, page 495, that its function is like that of the mam- 

 malian placenta, and, page 484, that, when fully formed, it consists, in 

 Salpa africana, of two separate chambers, a foetal chamber and a 

 maternal chamber. The lower chamber, which is identical with the 

 cul-de-sac of earlier stages, and is therefore part of the blood system of 

 the chain-salpa, is the maternal placenta. It opens below into the blood 

 space of the chain-salpa, and its roof, which separates it from the upper 

 or foetal chamber, is derived from the lower part of the follicle, and it 

 carries the "bell-clapper" pendant from its center. Its outer edges are 

 continuous with the side walls of the placenta. 



He says that the fostal chamber is at first in communication with 

 the digestive cavity of the embryo, and that its roof is to be traced back 

 to a saddle-shaped layer of endodermal blastomeres, which, at a very early 

 stage, separates from the embryo. In transverse sections this layer is 

 found to be made up of long cylindrical cells with many nuclei which 

 increase in length towards the top. These cells are identical with the 

 " germoblastic cells" of Todarro. At their inner ends these cells soon 

 begin to degenerate, and to give rise to the " yellow bodies " and " ovi- 

 form bodies " of Todarro, which latter are derived from the tissues of the 



