II.] THE ALLAXTOIS. 41 



the alimentary caual. "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 con- 

 tinuous 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. 8, H, I, &c. a.), and the fluid which it afterwards con- 

 tains 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 forma- 

 tion, 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 investment of the yolk-sac described in the 

 preceding paragraph. 



9. 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 difficulty 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 fundamentally an appendage of the 

 alimentary canal, and may be regarded as a bud thrown 

 out by the splanchnopleure close to its junction with the 

 somatopleure at the hinder end of the embryo (Fig. 8, D, 

 at.). From thence it grows first into the pleuroperitoneal 

 cavity of the embryo, and thence very rapidly pushes its 

 way by the development of a long stalk into the space 

 between the true and false amniotic sacs (Fig. 8, G, K). 

 Curving over the embryo, it comes to lie over the embryo 

 and the amnion proper, separated from the shell (and vitelline 

 membrane) by nothing more than the thin false amnion. 

 In this position it performs its functions as a respiratory 

 organ. It is evident that though now placed quite outside 

 the embryo, the space in which it lies is a continuation of 

 that peritoneal cavity in which it took its origin. 



It is only necessary to add, that the false amnion either 

 coalesces with the vitelline membrane, in contact with which 



