INTERRELATIONS OF THE MESONEPHROS, ETC. 197 



fetal ectodermal epithelium. The character of this epithelium 

 and its relations to the fetal capillaries, by which the material 

 can be carried to or from the fetus itself, is of importance in this 

 study, as it is through thin plates in this epithelium, closely 

 covering the fetal capillaries, that osmosis of urinaiy matter 

 would necessarily take place, if the present thesis is correct. 

 Once passed from the fetal blood to the maternal circulation, 

 these materials could be finally excreted by the maternal kid- 

 neys. 



In certain placentae the extreme thinness of this epithelial 

 layer and its close relation to the endothelium of the fetal capil- 

 laries are features of such prominence that they have been noted 

 and figured by every writer on the subject. This is the case 

 in the rodents. Duval, Minot, Grosser, all agree that in the 

 placental labyrinth of these animals the earlier thick tropho- 

 dermic layer becomes reduced to a very thin syncytium, with 

 thicker areas containing nuclei, on one side of which is the mater- 

 nal blood, on the other side the fetal capillaries (figs. 6, 7, and 

 8). In fact Duval- goes further and reports the ultimate disap- 

 pearance of the syncytial plates. The capillaries thus appeared 

 to him naked in the maternal blood-stream. The other authors 

 do not agree with this, and, I think, rightly; it is much more 

 likely that the plates and the endothelium are so intimately asso- 

 ciated as to appear a single layer, as in the renal glomeruli, 

 where they have been proved by Drasch to retain their inde- 

 pendence, as already stated alcove. The conception of the nu- 

 cleated portions of the syncytium remaining in situ but isolated 

 from each other is rather hard to grasp. 



The rodent placenta is thus pro^'ided with a membrane pre- 

 sumably suitable for osmosis as well as thicker nucleated por- 

 tions for active secretion or absorption. But the different types 

 of rodents differ in the length of time necessary to attain this 

 arrangement. In the rabbit this modification of the tropho- 

 derm, "la periode d'achevement de I'ecto-placenta," according 

 to Duval, comes near the middle of pregnancy, at 25-to 30 days. 



« Duval, loc. cit., p. 119. 



THE AMEIUC-VN' JOl.liVM, OF AX.VTOMV, VOI,. 1!), NO. 2 



