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 1s 
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 
