Still later the allantois grows forward, and becomes a large spherical vesicle, still however remaining connected with the cloaca by a narrow canal which forms its neck or stalk (fig. 121 G, _al_). From the first the allantois lies in the pleuroperitoneal cavity. In this cavity it grows forwards till it reaches the front limit of the hindgut, where the splanchnopleure turns back to enclose the yolk-sack. It does not during the third day project beyond this point; but on the fourth day begins to pass out beyond the body of the chick, along the as yet wide space between the splanchnic and somatic stalks of the embryo, on its way to the space between the external and internal folds of the amnion, which it will be remembered is directly continuous with the pleuroperitoneal cavity (fig. 121 K). In this space it eventually spreads out over the whole body of the chick. On the first half of the fourth day the vesicle is still very small, and its growth is not very rapid. Its mesoblast wall still remains very thick. In the latter half of the day its growth becomes very rapid, and it forms a very conspicuous object in a chick of that date (fig. 118, _Al_). At the same time its blood-vessels become important. It receives its supply of blood from two branches of the iliac arteries known as the allantoic arteries[68], and the blood is brought back from it by two allantoic veins which run along in the body walls (fig. 119) and after uniting into a single trunk fall into the vitelline vein close behind the liver. [68] I propose to call these arteries and the corresponding veins the allantoic arteries and veins, instead of using the confusing term 'umbilical.' [FIG. 124. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BIRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. _ep._ epiblast; _Sp.c._ spinal canal; _ch._ notochord; _n.e._ neurenteric canal; _hy._ hypoblast; _p.a.g._ postanal gut; _pr._ remains of primitive streak folded in on the ventral side; _al._ allantois; _me._ mesoblast; _an._ point where anus will be formed; _p.c._ perivisceral cavity; _am._ amnion; _so._ somatopleure; _sp._ splanchnopleure.] Before dealing with the later history of the foetal membranes, it will be convenient to complete the history of the yolk-sack. Yolk-Sack. The origin of the area opaca has already been described. It rapidly extends over the yolk underneath the vitelline membrane; and is composed of epiblast and of the hypoblast of the germinal wall continuous with that of the area pellucida, which on the fourth day takes the form of a more or less complete layer of columnar cells[69]. Between the epiblast and hypoblast there is a layer of mesoblast, which does not extend as far as the two other layers. The yolk is completely surrounded by the seventh day. [69] Further investigations are required as to the character of this layer.