748 GENERATION AND DEVELOPMENT. 



the amnion and the allantois, begin to be formed the am- 

 nion being developed by the external, and the allantois by 

 the internal layer of the blastodermic membrane. 



The amnion is produced in the fol- 

 lowing manner : The external layer 

 of the blastodermic membrane is raised 

 up in the form of a fold around the 

 body of the embryo, so that the latter 

 appears as if sunk in a kind of de- 

 pression, with the outer layer of the 

 membrane raised up wall-like around 

 it. On section, the appearance is that represented in 

 fig. 221. 



Soon the edges of the fold rising higher and higher 

 above and around the embryo, coalesce over it ; and the 

 double layer of membrane at their place of junction being 

 absorbed, the two layers of which the fold was originally 

 made up are separated from each other (figs. 223, 224) . The 

 inner of the two forms the amnion, and remains continuous 

 with the integument of the feetus at the umbilicus ; while 

 the outer layer, receding farther and farther, is fused and 

 forms one with the inner surface of the original viteliine 

 membrane, which in the meantime has undergone various 

 alterations to be immediately described (p. 751). 



As the term of pregnancy advances, the amnion becomes 

 more and more separated from the body of the foetus by a 

 considerable quantity of fluid, the so-called liquor amnii. 



During the process of development of the amnion, the 

 aUantois (c, fig. 222) begins to be formed. Growing out 

 from or near the hinder portion of the intestinal canal, with 

 which it communicates, it is at first a pear-shaped mass of 

 cells ; but becoming vesicular and very soon simply membra- 

 nous and vascular, it insinuates itself between the amniotic 

 folds, just described, and comes into close contact and 



* Fig. 221. Diagram of fecundated egg (after Dalton). a, um- 

 bilical vesicle ; &, amniotic cavity ; c, allantois. 



