200 JOHN LEWIS BREMER 



of excretion in man, already noted in the paucity and small size 

 of the mesonephric glomeruli. That they are actually present is 

 shown in figures 5, 9, 10, and 11, drawings of the chorion and 

 villi of the human placenta at different ages. Their extent in 

 relation to the blood-vessels is also shown in figure 12, the draw- 

 ing of a model of a placental villus from the placenta at term, 

 for which 1 am indebted to Mr. Alan Gregg, a student of this 

 School. The relation of fetal capillary, plate, and maternal 

 blood stream is the same as in the rodent placenta, and that of 

 capillary and plate the same as in any glomerulus. As in the 

 glomerulus, here also the plates are sometimes obviously separate 

 from the endothelium, sometimes so closely adherent as to appear 

 fused with it into a single layer. The stretches of the thicker, 

 granular protoplasm intervening between adjacent plates are 

 longer in the placenta than in the glomerulus, because the cells, 

 presumably of granular type, which supply nutrition, etc., to the 

 fetus are present in the placental epithelium, but absent in the 

 glomerulus. 



The time of the appearance of these plates in the human pla- 

 centa agrees sufficiently accurately with the time of the pro- 

 gressive degeneration of the mesonephric glomeruli in the embryo. 

 They are found, as is shown in figure 5, in the chorion of the 

 placenta of a fetus of 29.0 mm. Felix, as we have seen, places 

 the end of mesonephric involution at 22.0 mm., which should be 

 advanced slightly according to my own observations. The 

 plates in the placenta may well be present even earlier; I have 

 made no systematic search for them in closely graded stages. 

 The plates increase progressively in number as pregnancy ad- 

 vances, and are quite striking at term. Their location seems to 

 be gradually changed from the surface of the chorion in younger 

 placentae to the smaller villi in older ones, which means, of 

 course, that the early plates degenerate as new ones are constantly 

 being formed. 



We have seen that the placentae of all of that class of embryos 

 in which we have found an early involution of the mesonephros 

 exhibit membranous plates in the proper relation to the fetal 

 vessels, and that the time of the disappearance of the one is 



