EMBRYOLOGY. 131 
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 mem- 
brane unites with it, rises up into two folds (Fig. 169, Z). 
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, 2), 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 communicátion 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 Verte- 
brata grow for some time in the same manner. As devel- 
opment 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- 
