INTERRELATIONS OF THE MESONEPRHOS, ETC. 183 



The possibility that some other fetal organ might assume the 

 excretory function during the interim when neither the mesoneph- 

 ros nor the kidney is apparently capable of activity, or might 

 even in some animals like the rat and the mouse replace the 

 Wolffian body partially or entirelj^ is not considered by either 

 Weber or Felix; yet that this is the case, and that this organ is 

 the placenta seems to me to be strongly suggested by the facts 

 brought out in this paper. Among physiologists the ability of 

 the placenta to assume the role of the kidney is an accepted idea, 

 derived from many experiments on the permeability of the pla- 

 centa to a variety of substances passing from the fetus to the 

 mother. Wertheimer, in Richet's Dictionnaire de Physiologic, 

 published in 1904, mentions most of these experiments and 

 concludes that "the activity of the kidney does not appear to 

 be a function absolutely necessary during intra-uterine life; 

 the excretory products which form in the fetal organism could 

 be eliminated by the placental exchanges." Yet in this state- 

 ment he was forced to overlook the contradictory evidence of cer- 

 tain physiologists, who had found that the placental permeability 

 might not be the same in the earlier part of pregnancy as in the 

 later days, and that, even in the same stages of intra-uterine 

 life, one class of animals might differ from others in the results 

 obtained. Thus Krukenberg found that materials which passed 

 easily through the placentae of guinea pigs and rabbits were re- 

 tained in dogs and cats. Since Krukenberg, comparative re- 

 searches on the different classes of animals have not been under- 

 taken, all experimenters relying with a singular unanimity on 

 the results obtained from guinea pigs, rabbits, and man; but 

 these exceptions to the general rules are very interesting, in 

 the light of this present paper, as their explanation is simple, 

 once the facts of the different methods of fetal excretion in the 

 different animal classes are set forth. 



It is my purpose first to show anatomically the presence in 

 the placentae of certain animals of an excretory organ capable 

 of serving the fetus when neither the Wolffian body nor the kid- 

 ney is in activity, and second to point out the differences in res- 

 pect to their excretory activity in the types of animals studied. 



