8 7 4 



THE PLACENTA. 



amnion. It becomes closely applied to the inner surface of the chorion, and 

 extends even into its villi. The allantois proceeding from the umbilicus 

 comes to lie directly in contact with the fcetal membrane ; its sac disappears about 

 the 2nd month in man, but its vascular layer grows rapidly and lines the whole of 

 the inner surface of the chorion, where it is found on the 18th day (Coste). From 

 the 4th week the blood-vessels, along with a covering of connective-tissue, branch 

 and penetrate into the hollow cavities of the villi, and completely fill them. At 

 this time the primitive chorion disappears. Thus, we have a stage of general vascu- 

 larisation of the chorion. In the place of the derivative of the zona pellucida we 

 have the vascular villi of the allantois, which are covered by the epiblastic cells 

 derived from the false amnion. This stage lasts only until the 3rd month, when 

 the chorionic villi disappear all over that part of the surface of the ovum which is 

 in contact with the decidua reflexa. On the other hand, the villi of the chorion, 

 where they lie in direct contact with the decidua serotina, become larger and more 

 branched. Thus, there is distinguished the chorion laeve and c. frondosum. 



The chorion laeve, which consists of a connective-tissue matrix covered externally by several 

 layers of cells, has a few isolated villi at wide intervals. Between the chorion and the amnion 

 is a gelatinous substance (membrana intermedia) or undeveloped connective-tissue. 



Placenta. The large villi of the chorion frondosum penetrate into the tissue of 

 the decidua serotina of the uterine mucous membrane. [It was formerly supposed 

 that the chorionic villi entered the mouths of the uterine glands, but the researches 

 of Ercolani and Turner have shown that, although the uterine glands enlarge during 

 the early months of utero-gestation, the villi do not enter the glands. The villi 

 enter the crypts of the uterine mucous membrane. The glands of the inner layer 

 of the decidua serotina soon disappear.] As the villi grow into the decidua 



serotina, they push against 

 the walls of the large blood- 

 vessels, which are similar to 

 capillaries in structure, so that 

 the villi come to be bathed by 

 the blood of the mother in the 

 uterine sinuses, or they float 

 in the colossal decidual capil- 

 laries (fig. 665, VII, b). The 

 villi do not float naked in the 

 maternal blood, but they are 

 covered by a layer of cells 

 derived from the decidua. 

 Some villi, with bulbous ends, 

 unite firmly with the tissue of 

 the uterine part of the placenta 

 to form a firm bond of connec- 

 tion. [The placenta is formed 

 by the mutual intergrowth of 

 the chorionic villi and the de- 

 cidua serotina.] Thus, it con- 

 sists of a foetal part, including 



u i ,. i m*?'- 3 , , a ^ tne v iNi> and a maternal 



Human placental vilh. Blood-vessels black. or nterine ^ which is the 



very vascular decidua serotina. At the time of birth, both parts are so firmly 

 united that they cannot be separated. Around the margin of the placenta is a 

 large venous vessel, the marginal sinus of the placenta. [Friedlander found the 

 uterine sinuses below the placental site blocked by giant cells after the 8th month 

 of pregnancy. Leopold confirms this, and found the same in the serotinal veins.] 



