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.
The central nervous system (brain and spinal cord) is derived from the ectoderm by the development of a groove in the long axis of the embryonic area (Figs. 13, 14, 16 and 17), and by the subsequent union in the dorsal midline of the ridges bounding the groove to form a closed tube (Fig. 18). (Medullary groove, plates and canal.)
[Illustration: FIG. 13.--Transverse section of embryonic area of ovum of sheep of fourteen and a half days. (Heisler, after Bonnet.)]
[Illustration: FIG. 14.--Germinal area of rabbit’s ovum. (Kollmann.)]
[Illustration: FIG. 15.--Surface-view of area pellucida of an eighteen-hour chick-embryo. (Balfour.)]
[Illustration: FIG. 16.--Transverse section of human embryo before development of protovertebræ or chorda dorsalis. (Keibel.)]
[Illustration: FIG. 17.--Transverse section of a sixteen and a half day sheep embryo. (Heisler, after Bonnet.)]
[Illustration: FIG. 18.--Embryo of bird, at beginning of third day, with four blastodermic layers, resulting from the division of the mesoderm into parietal and visceral layers, separated by the cœlom cavity. Transverse section. × 170. (Kollmann.)]
The following changes in the ventral aspect lead to the formation of the alimentary canal and body-cavity:
The developing embryo at first lies flat on the subjacent yolk-mass, and subsequently becomes gradually separated more and more from the rest of the blastoderm by grooves or furrows which develop along the sides and at the cephalic and caudal extremity of the embryo. The folds resulting from these furrows indent the yolk more and more as development proceeds and tend to approach each other at a central point, the future _umbilicus_.