22b' NORMAL HISTOLOGY. 



become enlarged, so that the whole mucosa is greatly thickened. 

 Later in the course of pregnancy compression by the growing foetus 

 and increased amount of amniotic fluid cause a condensation of 

 these tissues, which become more fibrous with flattened spaces repre- 

 senting the uterine glands, the lining epithelium of which becomes 

 diminished in size and flattened. These changes occur in those por- 

 tions of the mucous membrane, the decidua vera, which do not take 

 part in the formation of the placenta. The ovum, when it reaehes 

 the cavity of the uterus, becomes embedded in this tissue, which 

 grows around and encloses it, after which it is differentiated into 

 three portions. The part beneath the ovum is called the decidua 

 serotina ; that which invests the ovum, the decidua reflexa ; and 

 that lining the rest of the uterine cavity, the decidua vera. As the 

 ovum enlarges, the decidua reflexa comes in contact with the decidua 

 vera, and the two layers exert a mutual pressure upon each other, 

 which flattens the spaces they contain and may obliterate many of 

 them. These atrophic changes are most marked in the decidua 

 reflexa, which, according to some authors, entirely disappears or is 

 reduced to a thin layer blending with and indistinguishable from the 

 decidua vera. The disappearance of the superficial epithelium of the 

 mucosa removes what might otherwise be a certain means of distin- 

 guishing these layers. The decidual tissue now consists of a number 

 of flattened spaces which are separated from each other by thin walls 

 of fibrous tissue produced by the further development of the decidual 

 tissue. The decidua reflexa and the decidua vera blend with each 

 other to form a part of the membranes that are expelled from the 

 uterus, along with the placenta, after the birth of the child. The 

 placenta is formed in the decidua serotina, and the changes in this 

 portion of the uterine mucosa during pregnancy differ from those in 

 the deciduse vera and reflexa. Most of the tissues which constitute 

 the afterbirth at parturition are of fetal origin. The outer surface 

 of the ovum, when it reaches the uterus, is composed of a single 

 layer of ectodermal cells ; beneath this is a layer of embryonic 

 connective tissue, derived from the mesoderm and resembling mucous 

 tissue, with few if any fibres. These two layers of tissue form 

 the outer envelope of the ovum, the chorion. Through a prolifera- 

 tion of the ectodermal cells, little projections, or villi, are formed on 

 the surface of the chorion, and subsequently the mesodermal con- 

 nective tissue grows into these villi, so that it forms the central part 

 of each villus, covered with a single layer of ectodermal epithelium. 



