THE FCETAL MEMBRANES OF MAN. 259 



which is turned toward the embryo is concave (figs. 139 and 143) 

 and altogether smooth, since it possesses a covering of the amnion 

 (am) ; the surface which reposes on the wall of the uterus is convex, 

 after its detachment at birth feels uneven, and is divided by deep 

 furrows into separate lobes or cotyledons. 



The normal position of the placenta is, in the majority of cases, 

 at the fundus uteri, where it is sometimes developed more to the 

 left side, sometimes more to the right. Consequently the opening 

 of one or the other of the Fallopian tubes may be covered and sealed 

 by it. 



In rare cases the placenta, instead of being attached to the fundus, 

 is united to the wall of the uterus nearer its mouth [os uteri]. This 

 results from the fact that the fertilised egg, when it passes from the 

 Fallopian tube into the cavity of the uterus, sinks down farther 

 owing to abnormal conditions, instead of attaching itself at once to 

 the mucous membrane. 



Occasionally the attachment takes place quite low, in the immediate 

 vicinity of the inner mouth of the uterus. In this case, as the 

 placenta with the growth of the foetus extends itself, it grows either 

 partly or wholly over the mouth of the uterus, and closes it more 

 or less completely. This anomaly is known as placenta prcevia 

 (lateralis or centralis) and presents a dangerous condition, because 

 the regular progress of birth is disturbed. 



In consequence of the low position of the placenta perilous bleeding is pro- 

 duced, either during pregnancy, or at least at the beginning of labor pains, 

 because the placenta detaches itself from the wall of the uterus prematurely, 

 whereby large blood-vessels are ruptured and laid open. 



In the investigation of the finer structure of the placenta serious 

 obstacles are encountered, since it is a very soft organ traversed by 

 numerous capacious blood-vessels. Therefore very contradictory views 

 still prevail concerning many points which are of the greatest 

 importance in judging of the structure. It does not appear to me 

 possible to give at present a final opinion upon these points. 



In the description it is best for us to start with the fact that the 

 placenta, as was previously stated, is composed of two parts, of one 

 part which is furnished by the embryo, and another part which is 

 produced by the mother, the placenta fcetalis and the placenta 

 uterina (Plate II.). 



The placenta fodalis is the part of the chorion (chorion frondosum) 

 which is thickly covered with much-branched villi. The villi (z), 

 united into great tufts or cotyledons, elevate themselves from a firm 



