CHAP, ii.] PREGNANCY AND BIRTH. 1515 



glands and intervening connective tissue altered but yet extant, 

 gives way before complex changes, the details of which have been 

 and still are the subject of much discussion, changes by which the 

 whole region, stretching from the basal portion of the uterine 

 glands, or even from the uterine muscular coat, to the connective 

 tissue which carries the capillary loops in which the umbilical 

 arteries end, is so altered that it becomes difficult to say which are 

 maternal, which are embryonic elements, which structures are of 

 glandular and true epithelial origin, which of connective tissue or 

 epithelioid origin. 



952. When fully developed, the placenta is a cake-like mass, 

 more or less distinctly divided into lobes, or cotyledons, which 

 occupies normally the roof of the uterus between the mouths of the 

 Fallopian tubes. The umbilical vein with the twisted umbilical 

 arteries, supported by the jelly-like immature connective tissue, 

 ' Wharton's jelly,' of the umbilical cord, stretching upwards from 

 the underlying foetus (such being the name given to the embryo 

 when its development is advanced), reach the placenta at about its 

 middle, and radiate thence over its surface. 



In vertical sections taken through the uterine wall and placenta 

 the following structures may be observed. Next to the now greatly 

 hypertrophied and very vascular uterine muscular coat, separated 

 from it by a thin layer of connective tissue, comes a layer of a 

 cellular nature which possesses few blood vessels of its own, but 

 which is traversed by conspicuous arteries whose course is very 

 tortuous, " curling arteries," passing from the muscular coat to the 

 parts beyond the layer and by corresponding veins whose course 

 is straighter. This layer, of which the uterine surface is smooth, 

 but the other surface very irregular, marked with rounded 

 projections, is often spoken of as the "decidual layer" of the 

 placenta, and may be regarded as the transformed uterine mucous 

 membrane. But the transformation is very great. Some authors 

 recognize in it the basal remnants of the uterine glands ; these 

 however are greatly obscured by the presence of cells having 

 the appearance of epithelium cells, which may be spoken of under 

 the general name of decidual cells, but which appear to be not all 

 of the same kind, some of them being multinucleated giant cells, 

 and about the origin of which there is much diversity of opinion ; 

 indeed some authors maintain that the whole of the layer is a new 

 formation, not originating even in part from the uterine glands, 

 the whole of these having been absorbed to make way for it. 



Next to this decidual layer comes what may be called the 

 placenta proper. This, except on the under surface, where covered 

 by the amnion and supported by connective tissue are found the 

 larger branches of the umbilical vessels, is in the main made up of 

 two elements, namely the branching, in fact extremely arborescent 

 cauliflower-like foetal villi, and of the irregular spaces, intervillous 

 spaces left between the villi. Though the matter is one which has 



