A section through the trunk of the embryo on the fourth day is represented in fig. 119. The section passes through the region of the trunk behind the vitelline duct. The mesentery (_M_) is very much deeper and thinner than on the previous day. The notochord has become invested by a condensed mesoblastic tissue, which will give rise to the vertebral column. The two dorsal aortæ have now completely coalesced into the single dorsal aorta, and the Wolffian body has reached a far more complete development. In the course of the fifth day the face begins to assume a less embryonic character, and by the sixth and succeeding days presents distinctive avian characters. The general changes which take place between the sixth day and the time of hatching do not require to be specified in detail. _Foetal Membranes._ The Reptilia, Aves and Mammalia are distinguished from the Ichthyopsida by the possession of certain provisional foetal membranes, known as the amnion and allantois. As the mode of development of these membranes may be most conveniently studied in the Chick, I have selected this type for their detailed description. The Amnion. The amnion is a peculiar sack which envelopes and protects the embryo. At the end of the first day of incubation, when the cleavage of the mesoblast has somewhat advanced, there appears, a little way in front of the semilunar head-fold, a second fold (fig. 102, also fig. 121 C, _af_ and fig. 122, _Am_), running more or less parallel or rather concentric with the first and not unlike it in general appearance, though differing widely from it in nature. This second fold gives rise to the amnion, and is limited entirely to the somatopleure. Rising up as a semilunar fold with its concavity directed towards the embryo (fig. 121 C, _af_), as it increases in height it is gradually drawn backwards over the developing head of the embryo. The fold thus covering the head is in due time accompanied by similar folds of somatopleure, starting at some little distance behind the tail, and at some little distance from the side (fig. 121 C, D, E, F, and 116, _am_). In this way the embryo becomes surrounded by a series of folds of thin somatopleure, which form a continuous wall all round it. All are drawn gradually over the body of the embryo, and at last meet and completely coalesce (fig. 121, H, I, and 117, _Am_), all traces of their junction being removed. Beneath these united folds there is therefore a cavity, within which the embryo lies (fig. 121 H, _ae_). This cavity is the cavity of the amnion. [FIG. 121. A to N forms a series of purely diagrammatic representations introduced to facilitate the comprehension of the manner in which the body of the embryo is formed, and of the various relations of the yolk-sack, amnion, and allantois.