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 Salpa, but 

 they assume a fundamental similarity to its function in 

 mammals. 



So far as it is 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 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 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, wiiich 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 



