VAGINA. 193 



or projecting clusters of cells ; this epithelioid investment is derived 

 its deeper part at least from the epiblastic outer layer of the chorion ; 

 partly also, perhaps, from the epithelium of the uterine mucosa. The 

 twigs of the villi are separated by open spaces, " intervillous spaces," 

 into which the maternal arteries and veins in the decidua open 

 directly; these spaces thus form an extensive system of blood-sinuses, 

 filled with maternal blood, into which the chorial villi project. By 

 interchanges between the foetal blood circulating in the villi and the 

 maternal blood which surrounds the villi the alimentary and excretory 

 needs of the foetus are provided for. 



The decidua serotina, which is continuous with the decidua vera, 

 forms the deepest stratum of the placenta ; it is a thin lamina, and 

 has been already described as the mucosa of the pregnant uterus. 

 Among its constituents the abundant " decidual cells " are notable. 

 Septa from the decidua pass part way into the placenta and divide it 

 into lobes or cotyledons. At the margin of the placenta these 

 septa mav reach the chorion and form in places a " subchorial decidual 

 layer." 



The umbilical cord is a vascular cord connecting the foetus with the 

 placenta. It is formed by the stalk of the chorion and allantois, 

 covered on the surface by the amnion. It consists of two arteries and 

 one vein, twisted spirally, surrounded by gelatinous embryonic or 

 young connective tissue (" Wharton's jelly"), and lined superficially 

 with epithelium. The epithelium covering the surface of the cord is 

 of the squamous variety, stratified or simple, and is continuous with 

 the epithelium lining the amnion, also with the foetal epidermis. 



In the earlier part of pregnancy the jelly of Wharton or connective 

 tissue surrounding the umbilical vessels is a mucous tissue, composed 

 of anastomosing mucous cells with the meshes and interstices between 

 them filled with a soft mucinous or gelatinous intercellular substance; 

 toward the close of pregnancy fibrous elements develop abundantly in 

 the interstices of this tissue. The umbilical vessels possess thick walls, 

 made up mainly of laminae of involuntary muscle-cells arranged longi- 

 tudinally, obliquely, and transversely ; elastic fibres are scanty in the 

 vessel-walls, and the ad ventitia merges into the surrounding Wharton's 



J ell .v- 



At an early period the cord also contains the cavity of the allantois 

 and the stalk of the umbilical vesicle, both being tubules lined with 

 hypoblast ; these structures later disappear, though traces of them 

 may sometimes remain visible as columns or canals of epithelioid cells. 



The vagina : The walls of the vagina are composed of 

 mucous, submucous, muscular, and fibrous layers, from within 

 outward. The inner surface is disposed in transverse rugae. 

 The mucosa consists of stratified squamous epithelium resting 

 on a tunica propria. The latter contains abundant elastic 

 fibres and diffuse lymphoid tissue, and its surface exhibits 

 papillae projecting into the epithelial layer. No glands are 



13 Hist. 



