UTERUS. 257 



pregnancy a hyaline degeneration takes place (Minot), so that 

 it cannot be recognized at the end of pregnancy. According 

 to Leopold, however, it is fused with the decidua vera, and is 

 always to be seen. 



The decidua serotina (basalis) in the beginning has the 

 same structure as the decidua vera, but becomes complicated 

 in the course of pregnancy by the formation of the placenta. 



Placenta. 



The placenta usually is discussed in detail in the text- 

 books of embryology, but since it consists not only of an 

 embryonic part (placenta fcetalis), but also a maternal part 

 which is modified uterine mucosa (placenta uterina s. ma- 

 terna), a brief description must also be given here. 



The placenta fo3talis consists of a connective-tissue mem- 

 brane, the membrana c/wrii, which on the surface toward the 

 uterine wall possesses many richly branched villi. These give 

 rise to the name chorion frondowm, which is applied to the 

 membrane. The chorionic villi are grouped in large bundles 

 or cotyledons. After the third month the chorion comes in 

 contact with the second foetal membrane, the amnion, and later 

 on is connected closely with it. The amnion is a thin mem- 

 brane which consists of an epithelial and a connective- tissue 

 layer. The epithelial coat covers its free surface and lines the 

 whole amniotic cavity in the form of a single layer of flat- 

 tened cells. The connective-tissue sheath fuses with that of the 

 chorion. Through the umbilical cord there enter the mem- 

 brana chorii two umbilical arteries, which carry the blood of 

 the embryo to the placenta foetalis, where they branch freely. 

 To each cotyledon there runs one branch, which breaks up 

 into many twigs and forms capillary networks in the villi. 



A part of the villi end freely, while others pass into the 

 placenta uterina and become firmly connected with it. The 

 latter are called \he fastening villi or Haftwurzeln. By means 

 of these the two sides of the placenta are joined securely 

 together, so that in the later months of pregnancy no separa- 

 tion occurs. 



