PLATE 2 



EXPLANATION OF FIGURES 



5 Port ion of placental chorion of human embryo of 29.0 mm (H. E. C. 

 No. 389). Above, the chorionic mesoderm; the basal layer of the ectoderm and 

 the syncytial layer are both interrupted by a fetal capillary, separated from 

 the maternal blood stream only by an ectodermal plate, pi., which is closely 

 adherent to the endothelium of the capillary. X 640 diameters. 



8 Portion of chorion and labyrinth of placenta of a rat of 13 days (H. 

 E. C. no. 1930, sect. 143). The same production of epithelial plates separating the 

 endothelium of the fetal capillaries from the maternal l)lood stream. The two 

 streams are recognizable by their blood corpuscles. It will be noticed that the 

 plates occur against both fetal arteries and veins. The basal layer of fetal 

 ectoderm has partially disappeared. X 250 diameters. 



9 Villus of human placenta of 3 months. Note the complete syncytial 

 layei of the fetal ectoderm, and the basal layer interrupted by a fetal capillary, 

 ovei- which the syncytium has developed a plate. X 480 diameters. 



10 and 11 Villi of human placenta at term. The basal layer of ectoderm is 

 no longer present. The syncytial laj-er shows a succession of thick granular 

 nucleated portions and thin epithelial plates in direct contact with the fetal capil- 

 laries. The maternal blood stream surrounds the villi. X 480 diameters. 



12 Model of the blood-vessels and tlic ectodermal syncytium of a villus 

 of the human placenta at term. It will l)e noticed that two small villi have 

 fused, making a ring formation, around which capillaries pass. One artery and 

 two veins pass into the villus. In addition to the areas seen in profile where the 

 ectodermal covering is of plate-like thinness, the blood-vessels are also covered 

 b}- plates between x and x, and at y, and z. This, with figure 11, shows the rela- 

 tive extent of the plates and the thicker syncytium. X 250 diameters. 



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