46 PRELIMINARY ACCOUNT. [CHAP. 



fold of the amnion into the cavity which surrounds 

 the alimentary canal. When the several folds meet 

 and coalesce together above the embryo, they unite 

 in such a way that all their inner limbs go to form a 

 continuous inner membrane or sac, and all their outer 

 limbs a similarly continuous outer membrane or sac. 

 The inner membrane thus built up forms a completely 

 closed sac round the body of the embryo, and is called 

 the amniotic sac, or amnion proper (Fig. 9, H, J, &c. a.), 

 and the fluid which it afterwards contains is called 

 the amniotic fluid, or liquor amnii. The space between 

 the inner and outer sac, being formed by the united 

 cavities of the several folds, is, from the mode of its 

 formation, simply a part of the general cavity found 

 everywhere between somatopleure and splanchnopleure. 

 The outer sac over the embryo lies close under the 

 vitelline membrane, while its periphery is gradually 

 extended over the yolk as the somatopleuric invest- 

 ment of the yolk-sac described in the preceding para- 

 graph. It constitutes the false amnion while the mem- 

 brane of which it forms a part is frequently known as 

 the serous membrane. 



The Allantois. If the mode of origin of these two 

 sacs (the inner or true amnion, and the outer or false 

 amnion, as Baer called it) and their relations to the 

 embryo be borne in mind, the reader will have no diffi- 

 culty in understanding the course taken in its growth 

 by an important organ, the allantois, of which we shall 

 hereafter have to speak more in detail. 



The allantois is essentially a diverticulum of the 

 alimentary tract, into which it opens immediately in 

 front of the anus. It at first (Fig. 11, al) forms a 



