W. K. BROOKS ON THE GENUS SALPA. 51 



tion of the placenta of salpa is to bring about by osmosis an interchange 

 of fluids between the blood of the parent and that of the embryo, as in 

 the placenta of a mammal. 



The subject has received very little attention, but as no one has ever 

 commented upon the view set forth at considerable length by Leuckart 

 (1) pp. 61 and 62, this may be regarded as the accepted view. He says: 

 "The histological differentiation of the organs and tissues of the embryo 

 is accelerated, to a high degree, by the circulation in the body of the 

 young salpa, which is completely separated from the circulation of the 

 mother. At no time does the blood of the mother pass through the wall 

 of the placenta into the body of the embryo. The transfusion between 

 the mother and the foetus is, as in the mammals, purely endosmotic, 

 through the substance of the placenta, and it is most essentially facili- 

 tated by the movement of the blood, both in the embryo and in the chain- 

 salpa. 



" The upper wall of the placenta, which is the peculiar seat of the 

 process of diffusion, projects into the body of the embryo, and is sur- 

 rounded by the median ventral blood sinus. As the blood corpuscles of 

 the embryo are much smaller than those of the chain-salpa, it is easy to 

 see that no mingling takes place." 



It is probably true that no transfusion of blood corpuscles takes 

 place, and it is difficult to show from the study of sections of hardened 

 specimens that no serum from the blood of the chain-salpa is diffused 

 through the wall of the placenta, although its great thickness seems 

 to be a very unfavorable condition for this purpose, and I shall show 

 farther on that the mechanism of nutrition is very different from that of 

 mammals ; that this is effected by the actual migration of great placenta 

 cells, Plate XLV, Fig. 4, 20, into the body cavity of the embryo. The pla- 

 centa is an organ for the nourishment of the placenta cells by the blood of 

 the chain-salpa ; and the subsequent degeneration of these cells, after they 

 have migrated into the body of the embryo, supplies the material for 

 the growth of the embryo. This is in all probability the only function 

 of the placenta, for there does not seem to be any need for an especial 

 apparatus for oxidation, or for the removal of waste products. The 

 salpa embryo stands in much more direct relation to the external world 

 than the mammalian embryo. It projects into the cloaca of the chain- 

 salpa, and is freely exposed to the constant current of fresh sea-water 

 which flows around it, and its thin surface seems to be much more 

 favorable than the thick wall of the placenta for the diffusion of gases. 



