50 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



placenta, for the purpose of affording to the growing embryo an abundant 

 supply of nutriment. 



The resemblance between the fostal life of salpa and that of a mam 

 mal is most remarkable, and it is all the more noteworthy since we may 

 be absolutely confident that the placenta of salpa is an independent 

 acquisition, entirely without genetic relation to that of mammals. 



No modern writer except Todarro has ventured to regard the two 

 structures as homologous, and their phylogenetic independence is so 

 obvious that it is not necessary to discuss it, although a greater physio- 

 logical and anatomical resemblance than the facts warrant has usually 

 been assumed. 



We should hardly expect fundamental similarity in structures of 

 diverse origin. On the contrary, we might reasonably look for profound 

 differences between the placenta of salpa and that of the mammals. 



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 nevertheless 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 farther than this, and the way in 

 which the nourishment is conveyed to the embryo is totally unlike ; a 

 fact which has never been described nor 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 fostus 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 (4) p. 495, that the func- 



