The Af 



349 



membrane which, by contiuuing to grow upward and converge, 

 finally meets above the dorsal surface of the embryo and fuses. 

 By .the fusion there is formed a double sac which envelops the fetus 

 completely except at its point of origin at the ventral surface, where 

 it takes part in the formation of the umbilicus. The inner of these 

 two membranes constitutes the true amnion, while the external one 

 forms the external or false amnion which, fusing with or re- 

 placing the prochorion, forms the primitive chorion and later, 

 when] the allantois grows out, blends with it to contribute to the 

 formation of the permanent, or allantois- chorion. This outer, or 



Medul/ary 



Notochoyd. Sotiiite Gut entodertn. 



Fk;. 69. Transverse section of a sixteen-and-a-half- da}' sheep-embryo 

 with six somites. ( Heisler, after Bonnet). 



false, amnion is merely a portion of the external wall of the 

 blastodermic vesicle, without its relations having been changed 

 exteriorly. The .space between the amnion and the fetus con- 

 stitutes the amniotic cavity and is filled with the amniotic fluid. 

 As the amniotic cavity becomes filled with fluid and enlarges, 

 it closely invests the vitelline stalk and the allantoic cord in its 

 passage through this cavity. The amount of liquor amnii varies 

 greatly in different animals and at different periods of pregnancy, 

 but it is generally most abundant at about the middle of gesta- 

 tion. In the cow and mare the amount of amniotic liquid varies 

 from 5-6 liters, while, in the .sheep, according to St. Q.yx and 

 Violet, it varies between 100 and 500 grammes. In woman, the 

 amount is said to be about i % liters. 



