After segmentation is completed a cavity filled with fluid and surrounded by the developing cells is gradually formed in the interior of the mass. This cavity is known as the _segmentation-cavity_. The egg is now called the _blastula_, _blastosphere_ or _blastodermic vesicle_ and the cellular membrane enclosing the segmentation-cavity forms the _germinal membrane_ or _blastoderm_ (Figs. 5 and 6). The cells of the blastoderm become aggregated at one point on the circumference of the vesicle (dorsal pole of blastosphere) forming, when viewed from above, a thickened biscuit or disk-shaped opaque area. This is known as the _germinal area_, or _primitive blastoderm_ or _embryonic shield_ (Figs. 7 and 12). [Illustration: FIG. 5.--Blastodermic vesicle of rabbit. (After E. von Beneden.)] [Illustration: FIG. 6.--Blastodermic vesicle of _Triton taeniatus_. (Hertwig.)] [Illustration: FIG. 7.--Embryonic area of rabbit embryo. (Heisler, after E. von Beneden.) The primitive streak beginning in the cell-proliferation known as the "node of Hensen."] [Illustration: FIG. 12.--Oval embryonic area of rabbit's egg, detached with part of wall of blastodermic vesicle. x 30. (Kollmann.)] This is the first indication of the coming division of the entire egg-cell into the _embryo proper_ and the _vitelline_ or _yolk-sac_ (Figs. 8 and 9). The entire future individual develops from the cells of the germinal area. This area comprises both the embryo proper and the region immediately surrounding it. [Illustration: FIG. 8.--Blastodermic vesicle of mammal. (E. von Beneden.) The layer of cells lining the interior of the vesicle next to the zona pellucida forms Rauber's "Deckschichte" or prochorion. This is not the true ectoderm, since it does not participate in the formation of the embryo, which is entirely derived from the cells of the germinal area.] [Illustration: FIG. 9.--Human embryo with yolk-sac, amnion, and belly-stalk of fifteen to eighteen days. (Heisler, after Coste.)] The remainder of the ovum, serving temporary purposes of nutrition and respiration, gradually becomes absorbed and disappears. [Illustration: FIG. 10.--Embryonal area of sheep, composed of ectoderm and entoderm. (After Bonnet.)] [Illustration: FIG. 11.--Blastodermic vesicle of rabbit. Section through embryonic area at caudal limit of node of Hensen. (Rabl.)] Transverse sections at right angles to the long axis of the embryonic area show that the single layer of cells composing the primitive germinal membrane becomes differentiated first into two (Fig. 10) and subsequently into three layers of cells (Fig. 11). At the margins of the germinal area these layers are of course continuous with the rest of yolk-sac wall. From their position in reference to the center of the cell the three layers of the blastoderm are described as-- 1. The outer, Epiblast or Ectoderm. 2. The middle, Mesoblast or Mesoderm. 3. The inner, Hypoblast or Entoderm.