Each fold is necessarily formed of two limbs, both limbs consisting of epiblast and a very thin layer of mesoblast; but in one limb the epiblast looks towards the embryo, while in the other it looks away from it. The space between the two limbs of the fold, as can easily be seen in fig. 121, is really part of the space between the somatopleure and splanchnopleure; it is therefore continuous with the general space, part of which afterwards becomes the pleuroperitoneal cavity of the body, shaded with dots in the figure and marked (_pp_); so that it is possible to pass from the cavity between the two limbs of the amniotic folds 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 unite to form a continuous inner membrane or sack, and all their outer limbs a similarly continuous outer membrane or sack. The inner membrane thus built up forms a completely closed sack round the body of the embryo, and is called the amniotic sack, or _amnion proper_ (fig. 121, H, I, &c., _a_), and the fluid which it afterwards contains is called the amniotic fluid, or _liquor amnii_. The space between the inner and outer sack is, from the mode of its formation, simply a part of the general cavity found everywhere between somatopleure and splanchnopleure. The outer sack over the embryo lies close under the vitelline membrane, and the cavity between it and the true amnion is gradually extended over the whole yolk-sack. The actual manner in which the amniotic folds meet is somewhat peculiar (His and Kölliker). The head-fold of the amnion is the earliest formed, and completely covers over the head before the end of the second day. The side and tail folds are later in developing. The side-folds finally meet in the dorsal line, and their coalescence proceeds backwards from the head-fold in a linear direction, till there is only a small opening left over the tail. This also becomes closed early on the third day. The allantois[67] is essentially a diverticulum of the alimentary tract into which it opens immediately in front of the anus. Its walls are formed of splanchnic mesoblast with blood-vessels, within which is a lining of hypoblast. It becomes a conspicuous object on the third day of incubation, but its first development takes place at an earlier period, and is intimately connected with the formation of the posterior section of the gut. [67] For details on the development of the allantois the reader is referred to the works of Kölliker (No. 135), Gasser (No. 127), and for a peculiar view on the subject Kupffer (No. 136). In addition to these works he may refer to Dobrynin "Ueber die erste Anlage der Allantois." _Sitz. der k. Akad. Wien_, Bd. 64, 1871. E. Gasser, _Beiträge zur Entwicklungsgeschichte d. Allantois_, etc.