Section 28. Another Striking Contrast, Due to, or Connected With, This

plethora of yolk, is the differentiation of a yolk sac (= umbilical vesicle) and the development of two new structures, the amnion and allantois, in the fowl. If the student will compare Figure 10 of the frog, he will see that the developing tadpole encloses in its abdomen all the yolk provided for it. This is a physical impossibility in the fowl. In the fowl (Figure 2, Sheet 24) the enormous yolk (Y.) lies outside of the embryo, and, as the cells of the germinal area grow slowly over it, umbilical bloodvessels are developed to absorb and carry the material to the embryo. In the case of an embryo sinking in upon, as it absorbs, this mass of nutritive material, a necessity for some respiratory structure is evident. From the hinder end of the fowl's intestine, in a position corresponding to the so-called, urinary bladder of the frog, a solid outgrowth, the allantois, which speedily becomes hollow, appears. Early stages are shown in Figures 1 and 2, Sheet 24 (al.); while the same thing is shown more diagrammatically on Sheet 23, Figure 6 (all.). This becomes at last a great hollow sac, which is applied closely to the porous shell, and the extent of which will be appreciated by looking at Figure 5, Sheet 24, where the allantois is shaded. Allantoic bloodvessels ramify thickly over its walls, and aeration occurs through the permeable shell.