658 REPRODUCTION. 



pendicular. Immediately after penetrating the decidua, they dilate into 

 the placental sinuses (represented in the diagram in black), which ex- 

 tend through the whole thickness of the organ, embracing the ramifica- 

 tions of the foetal tufts. At this stage of completion the placenta is 

 essentially a vascular tissue. The other structures which originally 

 entered into its composition have disappeared, leaving only the blood- 

 vessels of the fcetus entangled with and adherent to the blood-vessels 

 of the mother. 



There is, however, no direct communication between the fcetal and 

 maternal vessels. The blood of the foetus is everywhere separated 

 from the blood of the mother by a thin partition, resulting from the 



FIG. 211. 



VERTICAL SECTION OF THE PLACENTA, showing the arrangement of the maternal and foetal blood- 

 vessels. a, a. Chorion. b, b. Decidua. c, c, c, c. Orifices of uterine sinuses. 



fusion of four different membranes, namely, first, the membrane of the 

 fcetal villus ; secondly, that of the uterine follicle ; thirdly, the wall of 

 the foetal blood-vessel ; and fourthly, the wall of the uterine sinus. But 

 this partition is of great extent, owing to the abundant ramification! 

 of the foetal vessels. The vascular tufts, in which the blood of the fcetus 

 circulates, are everywhere bathed, in the placental sinuses, with the 

 blood of the mother ; and the interchange of material between the two r 

 by absorption and exhalation, goes on with corresponding activity. 



It is easy to demonstrate the arrangement of the fcetal tufts in the- 

 placenta. They can be seen by the naked eye, and may be readily 

 traced from their attachment at the chorion to their termination near 

 the uterine surface of the placenta. The anatomical disposition of the 

 placental sinuses is more difficult of examination. During life, while 

 the placenta is attached to the uterus, they are filled with the blood of 

 the mother, occupying nearly or quite one-half the mass of the placenta. 



