THE ALLANTOIS 



347 



the outer forming the false amnion and the inner the true amnion. 

 The false amnion now coalesces with the original vitelline mem- 

 brane to constitute the false chorion. Evidently there is thus 

 formed a closed cavity, the amniotic cavity, between the true am- 

 nion and the body of the embryo. 



At first the amnion and the embryo are in close contact, but 

 soon the cavity begins to be distended with the fluid, the liquor 



FIG. 104. Diagram of 

 fecundated egg. 



a, umbilical vesicle; b, 

 amniotic cavity; c, allan- 

 tois. (Kirkes after Dai- 

 ton.) 



FIG. 105. Fecundated egg with allantois 

 nearly complete. 



a, inner layer of amniotic fold; b, outer layer of 

 ditto; c, point where the amniotic folds come in con- 

 tact. The allantois 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. 

 99, 100, in which will be found the structures external 

 to these folds. (Kirkes after Dalton.) 



amnii, which increases until it reaches a considerable quantity. 

 It affords mechanical protection to the fetus during intrauterine 

 life, and at labor serves to evenly dilate the cervix. When this 

 has been accomplished is the usual time at which the sac ruptures 

 and the liquor amnii escapes. It also supplies the fetal tissues 

 with water, parts of it being swallowed from time to time. 



The cavity between the false amnion and the true amnion is 

 continuous, with the body cavity at the umbilicus. 



Allantois. The allantois grows out from the back part of the 

 intestinal canal into the celom or the body cavity. (Figs. 104, 

 105). It is of splanchnopleuric origin. It soon becomes a mem- 



