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.)

Fig. 12.—Oval embryonic area of rabbit’s egg, detached with part of wall of blastodermic vesicle. × 30. (Kollmann.)

Fig. 13.—Transverse section of embryonic area of ovum of sheep of fourteen and a half days. (Heisler, after Bonnet.)

Fig. 14.—Germinal area of rabbit’s ovum. (Kollmann.)

Fig. 15.—Surface-view of area pellucida of an eighteen-hour chick-embryo. (Balfour.)

Fig. 16.—Transverse section of human embryo before development of protovertebræ or chorda dorsalis. (Keibel.)

Fig. 17.—Transverse section of a sixteen and a half day sheep embryo. (Heisler, after Bonnet.)