The Placenta 



361 



exchange of nutritive and waste products, but not of cellular 

 elements. 



The separation of the fetal from the maternal circulation is so 

 complete that most micro-organisms of disease do not ordinarily 

 pass through and, consequently, where the mother is affected 

 with a contagious disease, the fetus does not ordinarily contract 

 it through the medium of the blood. For example, in tubercu- 

 losis the fetus is almost never infected during its intra-uterine 

 existence, but is born free from the malady, however badly the 

 mother has been affected during the period of pregnancy. 



Fig. 79. Cotyledons of a cow, according to Colin. 



u. Uterus. Ch, Chorion. C, Maternal, C^, fetal portion of cotyledon. 

 Fetal and maternal portions are partly separated from each other. 

 (Bonnet.) 



The area, or areas, in the mucosa of the uterus at which elabo- 

 rate changes take place for the attachment and nutrition of the 

 fetus, is known as the maternal placenta and the corresponding 

 portion or portions of the chorion which sends capillary tufts 

 into the placental area of the uterus, constitute the fetal placenta. 



Among our domestic animals, there are great variations be- 

 tween these placentae. In some species, the relations existing 

 between the fetal villi and the capillaries of the uterus are so 

 intimate that, when the fetal placenta is removed, a portion of the 

 mucous membrane of the uterus is carried with it, while, in 

 other animals, the placental villi of the fetus separate from the 

 maternal tufts and leave them intact. Pfom this difference in 



