56 



EMBRYOLOGY 



tail end being from the first connected with the chorion by means of a thick bano; 

 of mesoderm, named the body-stalk (Bauchstiel) ; into this stalk the tube of the 

 allantois extends (Fig. 21). 



The Amnion. The amnion is a membranous sac which surrounds and protects 

 the embryo. It is developed in reptiles, birds, and mammals, which are hence 

 called "Amniota;" but not in amphibia and fishes, which are consequently termed 

 "Anamnia." 



In the human embryo the earliest stages of the formation of the amnion have not 

 been observed; in the youngest embryo which has been studied the amnion was 

 already present as a closed sac (Figs. 24 and 32), and, as indicated on page 46, 

 appears in the inner cell-mass as a cavity. This cavity is roofed in by a single 

 stratum of flattened, ectodermal cells, the amniotic ectoderm, and its floor consists 

 of the prismatic ectoderm of the embryonic disk the continuity between the 



roof and floor being established at 



Placental villi the margin of the embryonic disk. 



Outside the amniotic ectoderm is 

 a thin layer of mesoderm, which 

 is continuous with that of the 

 somatopleure and is connected by 

 the body-stalk with the meso- 

 dermal lining of the chorion. 



When first formed the amnion 

 is in contact with the body of the 

 embryo, but about the fourth or 

 fifth week fluid (liquor amnii) be- 

 gins to accumulate within it. This 

 fluid increases in quantity and 

 causes the amnion to expand and 

 ultimately to adhere to the inner 

 surface of the chorion, so that the 

 extra-embryonic part of the celom 

 is obliterated. The liquor amnii 

 increases in quantity up to the 

 _ . , , . sixth or seventh month of preg- 



KIQ. 28. . 



Yolk-sac 



Umbilical cord 



Allantois 

 Heart 

 Digestive tube 



Embryo 

 Amniotic cavity 



m illustrating a later stage in the development 

 of the umbilical cord. 



somewhat; at the end of preg- 

 nancy it amounts to about 1 liter. It allows of the free movements of the fetus 

 during the later stages of pregnancy, and also protects it by diminishing the risk 

 of injury from without. It contains less than 2 per cent, of solids, consisting of 

 urea and other extractives, inorganic salts, a small amount of protein, and frequently 

 a trace of sugar. That some of the liquor amnii is swallowed by the fetus is proved 

 by the fact that epidermal debris and hairs have been found among the contents of 

 the fetal alimentary canal. 



In reptiles, birds, and many mammals the amnion is developed in the following 

 manner: At the point of constriction where the primitive digestive tube of the 

 embryo joins the yolk-sac a reflection or folding upward of the somatopleure takes 

 place. This, the amniotic fold (Fig. 29), first makes its appearance at the cephalic 

 extremity, and subsequently at the caudal end and sides of the embryo, and grad- 

 ually rising more and more, its different parts meet and fuse over the dorsal aspect 

 of the embryo, and enclose a cavity, the amniotic cavity. After the fusion of the 

 edges of the amniotic fold, the two layers of the fold become completely separated, 

 the inner forming the amnion, the outer the false amnion or serosa. The space 

 between the amnion and the serosa constitutes the extra-embryonic celom, and 

 for a time communicates with the embrvonic celom. 



