586 



GENERATION AND DEVELOPMENT. 



latter appears as if sunk in a kind of depression, 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 



Fm. 221. 



FIG. 221. Diagram of fecundated egg (after Dalton). a, umbilical vesicle ; b, am- 

 niotic cavity ; c, allantois. 



FIG. 222. Fecundated egg with allantois nearly complete, a, inner layer of am- 

 niotic fold; b, outer layer of ditto; c, point where the amniotic folds come in con- 

 tact. The allautois is seen penetrating between the outer and inner layers of the 

 amniotic folds. This figure, which represents only the amniotic folds and the parts 

 within them, should be compared with Figs. 223, 224, in which will be found the 

 structures external to these folds. 



two layers of which the fold was originally made up are sep- 

 arated from each other (Figs. 223, 224). The inner of the two 

 forms the amnion, and remains continuous with the integu- 

 ment of the foetus at the umbilicus; while the outer layer, 

 receding farther and farther, is fused, and forms one with the 

 inner surface of the original vitelline membrane, which in the 

 meantime has undergone various alterations, to be immediately 

 described (p. 588). 



As the term of pregnancy advances, the amnion becomes 

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

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



During the process of development of the amnion, the allan- 

 tois (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 membranous and 

 vascular, it insinuates itself between the araniotic folds, just 

 described, and comes into close contact and union with the 

 outer of the two folds, which has itself, as before said, become 

 one with the external investing membrane of the egg. As it 

 grows, the allantois becomes exceedingly vascular, and in birds 



