298 OUTLINES OF CHORDATE DEVELOPMENT 



that in effect the chorion forms a part of the outer membrane 

 of the allantois. Along the dorsal surface of the embryo the 

 inner wall of the allantois meets the amnion, and the apposed 

 mesodermal layers of these two membranes fuse together, 

 forming a layer of muscle fibers which function for a time 

 similarly to those of the amnion, and then disappear. 



A modified region of the allantois should be mentioned here. 

 During the early days of incubation the albumen of the egg 

 condenses toward what may be described as the postero- 

 ventral side of the yolk, from which it is separated only by the 

 remains of the vitelline membrane, in the region where the 

 yolk-sac remains open (Fig. 116). About the ninth or tenth 

 day, the albumen, through continued loss of water, forms a 

 relatively small, dense mass, incompletely separated from the 

 yolk by the intervention of the yolk sac (Fig. 117). The 

 allantois reaches the albumen first from the lower side. It is 

 now fused with the chorion throughout its extent, and as it 

 pushes onward, it sends a short fold in between the albumen 

 and the yolk-sac, and a longer limb around the outer surface 

 of the albumen, almost covering it over. These folds of fused 

 chorion and allantois are the beginnings of the albumen sac. 

 The albumen sac is completed by the downgrowth of a 

 similar fold of fused chorion and allantois from the upper 

 side. These folds finally enclose the yolk completely (Fig. 

 117). The upper fold is not quite like the lower on account of 

 the obstruction offered to the progress of the allantois, by the 

 sero-amnionic fusion. When the rapidly extending allantois 

 reaches this area of fusion of the amnion and chorion, it pushes 

 the chorion out above the fused area, carrying a fold of the 

 chorion before it in its onward extension (Fig. 116). This 

 fold of the chorion, with the allantois within, is similar to the 

 outer limb of the ventral part of the albumen sac, but the 

 inner limb of the latter is consequently not represented on the 

 dorsal side. Dorsally the inner wall of the albumen sac is 

 formed only by the original chorion of the region, the sero- 

 amnionic connection being included (Fig. 117). About the 

 time the albumen sac is completed, by the fusion of the mem- 



