1120 THE PLACENTA. [Boon iv. 



tinguished as a new temporay lining to the uterus, the so- 

 called decidua. Into this decidua the ovum, on its descent from 

 the Fallopian tube, in which it has already undergone some 

 developmental changes, is received; and in this it becomes 

 embedded, the new growth closing in over it. Meanwhile the 

 rest of the uterine structures, especially the muscular tissue, 

 become also much enlarged ; as pregnancy advances a large 

 number of new muscular fibres are formed. 



As the ovum, now developing into the embryo and its 

 appendages, continues to increase in size, it bulges into the cavity 

 of the uterus, carrying with it the portion of the decidua which 

 has closed over it. Henceforward, accordingly, a distinction 

 is made in the now rapidly developing decidua between the 

 decidua reflexa, or that part of the membrane which covers the 

 projecting ovum, and the decidua vera, or the rest of the mem- 

 brane lining the cavity of the uterus, the two being continuous 

 round the base of the projecting ovum. That part of the 

 decidua which intervenes between the ovum and the nearest 

 uterine wall is spoken of as the decidua serotina. As the 

 embryo with its appendages continues to enlarge, carrying 

 with it the decidua reflexa, the latter becomes pushed against 

 the decidua vera, gradually obliterating the cavity of the uterus, 

 except at the cervix ; about the end of the third month, in the 

 human subject, the two come into complete contact all over, and 

 ultimately the distinction between them is lost. 



The decidua reflexa, and the decidua vera in general, undergo 

 various changes, but these are of subordinate interest compared 

 with those which take place in that part of the decidua vera 

 which is called the decidua serotina, and which lead to a special 

 union between maternal tissues and tissues belonging to the 

 growing embryo, a union which gives rise to the structure 

 known as the placenta or ' after-birth.' 



695. During the development of the ovum while some 

 of the cells, arising by cell-division from the primordial cell, 

 become the embryo proper, others form the appendages of the 

 embryo ; to the latter belongs the double bag which encloses 

 the embryo, and which consists of an inner bag, the true amnion 

 and an outer bag, the false amnion. The latter over the whole 

 of its surface is in contact with the decidua, and developes a 

 number of branched villi, consisting, like the rest of the mem- 

 brane, of an epithelium (epiblast) resting on a dermic (meso- 

 blastic) basis ; these villi are embedded in or applied to the 

 decidual surface. The false amnion, bearing villi, often called 

 the chorion, is at first devoid of blood vessels ; but a diver- 

 ticulum of the hinder part of the developing alimentary canal 

 of the embryo, called the allantois, grows out rapidly into 

 the space (containing fluid) between the false and the true 

 amnion, and soon applies itself to the former. As it grows, two 



