Nutrition of the Salpa Embryo. 371 
The various writers on Salpa, while recognizing this fact 
and while pointing out the great differences in the way in 
which the placenta is formed in the two cases, have never- 
theless assumed, either explicitly or by implication, a much 
greater resemblance to the mammalian placenta in structure 
and in function than actually exists. The later writers say 
very little about the function of the placenta of Sal/pa, but 
they assume a fundamental similarity to its function in 
mammals. 
So far as itis in both cases an organ for supplying the 
embryo with nutritive matter derived from the blood of the 
supporting organism, the resemblance is real; but it goes no 
further than this, and the way in which the nourishment is 
conveyed to the embryo is totally unlike, a fact which has 
never been described or even noted. 
In the mammalian placenta the blood of the embryo, as it 
circulates through the villi of the chorion, is brought into 
such close contact with the blood of the mother, that diffusion 
takes place through the separating walls, and thus the blood 
of the foetus is oxidized, relieved of its waste products, and 
supplied by diffusion with nutritive matter in solution. 
Notwithstanding the very intimate union between the 
blood-vessels of the foetus and those of the mother, there is no 
direct communication between them, and nothing except gases 
and liquids can pass from the body of the parent to the body 
of the child without the violent rupture or perforation of the 
walls of the vessels, unless perhaps some very minute Bacteria 
are an exception. 
It has been generally assumed that this must be true of 
Salpa also. ‘Thus, Barrois says incidentally and very briefly 
(p. 495) that the function of the placenta of Sa/pa 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 consider- 
able length by Leuckart (pp. 61 and 62), this may be 
regarded as the accepted view. He says:— The histolo- 
gical 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 facilitated 
