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TEXT-BOOK OF PHYSIOLOGY. 



becomes the primitive alimentary canal; the lower portion, remaining 

 outside of the body cavity, forms the umbilical vesicle. 



The Fetal or Embryonic Membranes. With the appearance 

 of the visceral laminae two membranes develop in succession, both 

 of which play an important part in the subsequent life of the embryo. 

 These are known as the amnion and the allantois. 



The amnion is formed by folds of the epiblast and the external 

 layer of the mesoblast rising up in front, behind, and at the sides. 

 These folds gradually extend over the back of the embryo to a certain 

 point where they meet, coalesce, and enclose a cavity known as the 

 amniotic cavity. The membranous partition between the folds is 

 absorbed, after which the outer layer recedes and becomes blended 

 with the primitive enveloping membrane of the ovum and thus 



FIG. 328. DIAGRAM OF FECUNDATED 

 EGG. a. Umbilical vesicle. b. 

 Amniotic cavity. c. Allantois. 

 (Dalton.} 



FIG. 329. FECUNDATED EGG WITH 

 ALLANTOIS NEARLY COMPLETE, a. 

 Innef layer of amniotic fold. b. 

 Outer layer of ditto, c. Point where 

 the amniotic folds, come in contact. 

 The allantois is seen penetrating 

 between the outer and inner layers 

 of the amniotic folds. (Dalton.) 



assists in the formation of the chorion the external covering of the 

 embryo. 



The cavity enclosed by the amnion is at first quite small, but 

 soon enlarges from the accumulation of a clear, transparent fluid, the 

 amniotic -fluid. It gradually increases in amount up to the latter 

 period of gestation, when its volume reaches about one liter. This 

 fluid is derived mainly from the blood, as it contains albumin, sugar, 

 fatty matter, and inorganic salts. Traces of urea indicate that some 

 of its constituents are derived from the embryo itself. 



The allantois is primarily a pouch or diverticulum which develops 

 from the posterior portion of the alimentary canal. As it develops 

 it enlarges, and in its growth inserts itself between the two layers 

 of the amnion, coming into contact more especially with the external 

 layer. It finally completely surrounds the embryo, after which its 

 edges become fused together (Figs. 328 and 329). 



