i628 A MANUAL OF ANATOMY 



further traversed by the allantoic stalk or belly-stalk, as that struc- 

 ture extends from the umbilical orifice of the embryo to the placental 

 area of the chorion, with which area the mesoderm of the allantoic 

 stalk blends, and to which area that mesoderm conducts blood- 

 vessels. 



The chorionic cavity becomes gradually obliterated as the 

 amniotic sac expands, the chorion and amnion then being intimately 

 related to each other. 



From the outer or ectodermic surface of the chorion small bud- 

 like processes grow into the contiguous maternal blood-lacunae. 

 These processes represent the rudimentary chorionic villi, and are 

 at first entirely composed of ectoderm. Subsequently the meso- 

 derm, which forms the inner layer of the chorion, grows outwards 

 into these rudimentary villi, and forms for each of them a core. In 

 this mesodermic core capillary bloodvessels are developed, which 

 are continuous with the allantoic vessels through the mesoderm of 

 the allantoic stalk, which fuses with the placental area of the 

 chorion. The villi are now complete, and are somewhat club- 

 shaped. A complete villus consists of the following parts: (i) An 

 external envelope of ectoderm, the superficial layer of which con- 

 sists of syncitium; (2) a vascular core of mesoderm; and (3) an 

 arteriole, capillaries, and a venous radicle. 



The villi at first beset the entire outer surface of the chorion, and 

 are of equal size. Subsequently, however, the villi of one particular 

 area undergo enlargement and ramifications, whilst the others 

 undergo retrogression, and become very small and scattered, ulti- 

 mately becoming atrophied. The area over which retrogression of 

 villi takes place becomes comparatively smooth, and this part of 

 the chorion is consequently called the chorion leve. The limited 

 area over which the villi become enlarged is rough and shaggy, and 

 is known as the chorion frondosum or leafy chorion. This area of 

 the chorion becomes intimately connected with the uterine mucosa, 

 usually at the upper part of the posterior wall of the uterine 

 cavity, close to the fundus. Along with the mesoderm of the 

 adjacent part of the allantoic stalk, it forms the foetal part of the 

 placenta. 



The villi of the chorion frondosum are numerous and large. 

 Moreover, they ramify and so become arborescent. Some of them 

 penetrate by filamentous processes into the uterine mucosa (decidua 

 basalis or serotina) in the placental region, and these processes, or 

 calkers, not only grip but become continuous with it. Such villi 

 are known as fixation- or root-villi. Others project into, and lie 

 free within, the maternal blood-lacunae, which are formed in connec- 

 tion with the decidua basalis or serotina of the uterine mucosa, 

 where they are freely bathed by the maternal blood. 



The uterine surface of the chorion frondosum has an epithelial 

 covering consisting of ectoderm. This ectoderm is disposed at an 

 early period in two layers — superficial and deep. The deep layer 

 is distinctly cellular, and is known as the layer of Langhans. The 



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