THE EARLY DEVELOPMENT OF THE CHICK 293 



ward over the head of the embryo, but before it has gone very 

 far the mesoderm, with the accompanying exoccelom, extends 

 into the fold, and the endodermal layer is withdrawn. The 

 lateral extremities of the head-fold of the amnion turn pos- 

 teriorly along the sides of the head, as the lateral folds of the 

 amnion. These are somatopleural from their beginning, and 

 may be described as elevations of the blastoderm in the region 

 where the outer limbs of the lateral embryonic folds pass into 

 the general surface of the blastoderm (Fig. 110). 



As these folds of the amnion extend posteriorly, the ectam- 

 nion serves actively to draw the more lateral regions toward the 

 mid-line where they are fused together, and in this way the 

 lateral folds are gradually drawn up over the embryo (Lillie). 

 This process occurs progressively in a posterior direction. 

 About the time the embryo has thus become covered over as 

 far back as the level of the vitelline arteries (48 hours, 26-28 

 pairs of somites), a posterior tail-fold of the amnion appears, 

 (Fig. 109, A), similar to its head-fold save that it is somato- 

 pleural, i.e., contains coelom and mesoderm, from its com- 

 mencement. This tail-fold extends laterally and then ante- 

 riorly, and soon its extremities meet the lateral folds. About 

 the seventieth hour (35 pairs of somites) the amnionic folds 

 are completely closed together above the embryo; this closure 

 occurs about at the level of the hind-limb buds. 



As the somatopleural amnion folds come together above the 

 embryo, the ectodermal and mesodermal layers of each side 

 fuse together and the original contact surfaces break through, 

 so that the ccelomic cavities of the amnionic folds become con- 

 tinuous across the mid-line. This results in the formation of 

 two complete membranes, each composed of two cell layers, 

 around the embryo. The inner of these, close to the embryo, 

 is the true amnion, and the space between this and the embryo 

 is the amnionic cavity (Figs. 119, 125). From the method of 

 its formation it will be seen that the amnion is continuous with 

 the somatic stalk, while the amnionic cavity is a space entirely 

 outside the embryo, and has no connection with any other 

 cavity. The outer membrane is now called the chorion. It is, 



