CHORION 



71 



the embryo floats. During the second month (fig. 97) it comes to fill the cavity 

 of the chorion, and the extra- embryonic cceiom is obliterated. As the umbilical 

 cord elongates, the amniou forms a tubular sheath round it, enclosing the vessels, 

 along with the allantoic and vitelline ducts, which are imbedded in mucous 

 connective tissue. From the cord it is reflected over the surface of the placenta 

 on to the chorion, with which it is intimately united to form what are known as 

 the foetal membranes. Between the amnion and chorion of the placenta lies the 

 umbilical vesicle. 



The flattened ectodermic cells of the amnion become cubical during pregnancy, 

 and at full time the membrane consists of an epithelial layer, and a lamella of 

 fibrous tissue. The epithelium has the form of a very regular layer of cubical 



FIG. 98. DIAGRAM OF A HUMAN OVUM AT A (HYPOTHETICAL) STAGE SOMEWHAT YOUNGER THAN 

 PETERS' OVUM ; IMBEDDED IN THE DECIDUA. (T. H. Bryce.) 



Th, blood-clot at point of entrance ; g, g, glands opening on the surface of the mucous membrane ; 

 d.c., decidua capsularis. The trophoblast partly cellular, partly plasmodial, is seen invading the 

 decidua, and opening up the dilated capillaries. The extravasated blood occupies the spaces or blood- 

 lacunse between the strands of trophoblast. The gland-tubules in the decidua take a concentric course 

 round the ovum. 



cells joined by distinct cell-bridges. The liquor amnii varies in amount at different 

 periods of gestation ; it is relatively most abundant about the fifth or sixth 

 month. In the later months of pregnancy it contains urea, which is probably 

 excreted by the kidneys of the foetus. 



Chorion. We have already seen in an earlier section that the formative cell- 

 mass, from which entoderm, as well as embryonic and amniotic ectoderm, are 

 formed, is completely surrounded in the primate ovum by a layer of cells which 

 has been named the trophoblast. In the youngest known ova the trophoblast 

 shows a very irregular outer surface (figs. 93 and 98) consisting of cellular strands 

 separated by spaces containing maternal blood. Round the wall of the vesicle 



