372 Mr. W. K. Brooks on the 



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 surrounded 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 unfavourable 

 condition for this purpose ; and I shall show further 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 into the body-cavity of the embryo. The 

 placenta is an organ for the nourishment of the placenta-cells 

 by the blood of the chain-Salpa, and the subsequent degene- 

 ration 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 favour- 

 able than the thick wall of the placenta for the diffusion of 

 gases. During the later stages of foetal life its own mouth is 

 open, its muscles contract, and there is no reason why it 

 should not breathe for itself exactly like an adult. I there- 

 fore regard the placenta as a nutritive organ pure and simple, 

 and it serves its purpose not by the diffusion of a fluid, but 

 by the transportation of solid food into the body of the 

 embryo. From this point of view it is clear that those 

 investigators who have described it as divided into a foetal 

 chamber and a maternal chamber have been misled by an 

 erroneous notion of its function. 



The detachment of the placenta-cells has been observed 

 and noted by both Salensky and Barrois, but it has been 

 regarded as a destructive change and as a sign that the organ 

 has served its purpose and has become superfluous. 



It has been assumed that it reaches its perfect form and 

 serves its purpose, and that it then degenerates and breaks 



