umbilical vesicle or yolk-bag. The gelatinous mass which surrounds the egg of the Frog furnishes the nutriment for the embryo. The development of the Reptile, Bird, and Mammal offers a striking contrast as compared with that of the Fish and Batrachian in the formation of the Amnion and Allantois. The External blastodermic membrane, at that point where the upper part of the Middle membrane unites with it, rises up into two folds (Fig. 169, d). These folds grow towards each other, arching over the embryo, and finally unite (Fig. 170). The inner fold then separates from the outer, and forms the Amnion (Fig. 171, d), while the outer fold recedes from the Amnion until it reaches the Vitelline membrane, with which it unites. These united membranes are known as the Chorion (Fig. 171, Ch). The Amnion becomes filled with the Amniotic fluid, in which the embryo is suspended. During the formation of the Amnion there buds out from the posterior portion of the embryo a sac (Figs. 169 to 172), which, in expanding, finally comes in contact with the Chorion, This sac is called the Allantois, and serves in Birds and Reptiles as a respiratory organ, the porosity of the egg-shell allowing the oxygen to pass in and the carbonic acid to pass out. In the Mammals, through the Allantois, the embryo is put in communication with the mother. We have now explained as briefly as possible the development of a vertebrate.
In the hatching process the Chorion, Allantois, and Amnion break, they being only temporary structures. It will be seen, therefore, that the animal is formed of but a portion of the three blastodermic membranes. Beginning alike in the form of a cell or egg, the Invertebrata and Vertebrata grow for some time in the same manner. As development advances, characteristic structures appear in the embryo, and the division, class, or order to which the future animal will belong becomes evident. Figs. 178 to 181, repre-
