vi PEEGNANCY PAETUEITION PUEEPEEIUM 225 



consequence of the rupture of the superficial vessels of the 

 compact stratum of the decidua. 



Other observers, on the contrary, are of opinion that perfect 

 cohesion, leaving no interstices of any kind, is established from 

 the first between the surface of the chorion and that of the 

 decidua. At successive stages of the evolutionary process, ample 

 vascular sinuses are formed in the compact stratum of the decidua, 

 causing it to assume the structure of cavernous tissue : the 

 chorionic villi are covered by a sheath of endothelium, which 



Fio. 90. Structure of the placenta ; diagram after Leopold ; edge of the placenta on the left. 

 P, decidua placentalis ; ds, stratum spongiosum of the decidua ; dc, stratum compactum of 

 the decidua; sch, subchorionic obturator ring; ss, decidual inteicotyledonal septa; r, line 

 along which the splitting of the decidua and hence the detachment of the placenta in 

 parturition is effected ; B, vascular layer of Winkler ; ch, chorion with chorionic villi ; v, free 

 prolongation of villus ; if, adherent prolongation of villus ; Am, amnion ; a, uterine artery 

 opening into the intervillous spaces, c. 



represents the wall of the placental sinuses, hence the appearance 

 of intervillous spaces filled with blood (Fig. 90). 



Whatever may be the truth with regard to this debated point, 

 it is certain that an intimate relation is established between the 

 villi of the chorion and the lacunae in which the maternal blood 

 circulates. This relation is the fundamental condition in the 

 development of the ovum by means of the exchange of the 

 nutritive materials and gases of respiration which are indispens- 

 able to the foetus, without any intermingling of maternal with 

 foetal blood taking place, as was believed in ancient times. When 

 the placenta is completely developed, it presents the appearance 

 of a circular or oval disk having a diameter of 13-20 cms. and 

 weighing on an average 500-600 grms. This estimate Sfameni 



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