REPRODUCTIVE ORGANS IN THE FEMALE. 331 



present, as maternal blood ruptures this wall. The 

 second investment, or syncytium, disintegrates. 

 The third also disappears, at least in parts, or be- 

 comes so thin that it can scarcely be detected. Ulti- 

 mately, therefore, the fetal and maternal blood is 

 practically separated by only one membrane, the 

 epithelium of the fetal capillaries. 



The maternal placenta does not differ structurally 

 from the histology of the decidua already described 

 except in degree of complexity. There is an inter- 

 nal compact portion and a deeper spongy layer. The 

 latter rests against the uterine muscularis and is very 

 vascular. Numerous blood- 

 vessels penetrate the compact 

 layer to open freely into the 

 intervillus sinuses already de- 

 scribed. These blood-vessels 

 usually take a very tortuous 

 course, and they are thus able 



to adjust themselves to contractions and expansions 

 of the uterine wall. Decidual cells are especially 

 conspicuous in the compact layer of the placenta. 

 These cells are sometimes present in the spongy 

 layer but never in the chorionic villi or fetal portions 

 of the placenta. 



The fetal blood reaches the placenta through the 

 umbilical cord. There is regularly present two ar- 

 teries and one vein, imbedded in a gelatinous con- 

 nective-tissue matrix known as Wharton's felly. 

 The blood in the arteries is carried to the placenta 

 and is venous. That in the vein returns from the 

 placenta and is arterial. After birth, when the pla- 



