340 



HUMAN UTERUS AND FCETAL APPENDAGES. 



a spongy mass containing a large quantity of blood and bounded by two mem- 

 branes, each less than a millimeter thick; the upper one is the chorion, covered 

 by the still thinner amnion, and greatly thickened where the vessels lie in it; the 

 lower one is the decidual tissue, together with the ends of the villi imbedded in 

 it (cf. especially page 335 and Fig. 194) ; it represents only a portion of the deci- 

 dua, the other portion having remained upon the uterine wall. The spongy mass 



Fig. 197.- — Human Placenta after Delivery at Full Term.' 

 A, Vertical section through the margin : D, decidua ; </, aborted villi outside the placenta ; Clio, chorion ; Si, 

 sinus; li, placental villi ; Fit, fibrin. B, Portion of A more highly magnified to show the decidual tissue 

 near b : ->, blood-vessel; d, decidual cell with one nucleus; r/', decidual cell with several nuclei. 



is found upon examination to consist of an immense number of tufts of fine rods 

 of tissue, which are irregularly cylindrical in shape. Further examination 

 shows that they are twigs (Fig. 204), with rounded ends and springing from 

 branclilets which in their turn arise from branches, and so on until a large main 

 stem is found, which starts from the chorion. This branching system is richly 

 supplied with blood from the fcetal vessels on the surface of the placenta. The 



