DEVELOPMENT OF GALLUS 213 



ectoderm of this region is continuous with and forms part of 

 the chorion ; the endoderm, continuous with the endoderm 

 of the embryo, now contains the yolk, and is known as the 

 yolk-sac. There is a layer of mesoderm on the inner side of 

 the ectoderm, and another on the outer side of the yolk-sac, 

 so that the extra-embryonic coelomic cavity extends down into 

 this region. The yolk-sac contains the store of nourishment 

 for the developing embryo, and the yolk digested is brought 

 into the embryo by the blood-vessels of the area vasculosa. 

 The yolk-sac represents the heap of yolk-cells in the hinder 

 part of the gut of the developing frog, but there is so much 

 yolk that it cannot be accommodated inside the cavity of the 

 gut as in that animal. Instead, it hangs in a sac beneath the 

 gut, and gets gradually drawn up as its contents diminish, 

 until, right at the end of development, it passes up through 

 the umbilicus into the intestine of the embryo. Just before 

 doing so, the albumen surrounding the chorion becomes con- 

 tained in a sac formed by folds of the latter. This sac com- 

 municates with the base of the yolk sac through the aperture 

 which was left open, and it communicates also with the 

 amniotic cavity by a reopening of the sero-amniotic connexion. 

 The amniotic fluid and albumen are therefore able to pass 

 into the yolk-sac and get absorbed. 



Allantois. — Shortly after the formation of the amnion a 

 median ventral downgrowth is developed from the floor of the 

 hind gut. This endodermal sac, covered on the outside by 

 a layer of mesoderm, is the allantois and it represents the 

 bladder of the frog. The allantois grows out into the extra- 

 embryonic coelomic cavity, and its size increases as that of 

 the yolk-sac diminishes. It soon occupies almost the entire 

 space within the chorion which is not filled by amnion and 

 yolk-sac. Its outer wall becomes applied to the inner surface 

 of the chorion, and the extra-embryonic coelomic cavity between 

 them disappears by the fusion of the two layers of mesoderm 

 (on the inner side of the chorion and on the outer side of the 

 allantois). This fused layer of allantois and chorion now 

 lies close against the inner surface of the shell, only separated 

 from it by the shell-membranes ; it is also highly vascular 



