of the tube forms the medulla spinalis or spinal cord; from its ectodermal wall the nervous and neuroglial elements of the medulla spinalis are developed while the cavity persists as the central canal.
THE NOTOCHORD.
The notochord (Fig. 19) consists of a rod of cells situated on the ventral aspect of the neural tube; it constitutes the foundation of the axial skeleton, since around it the segments of the vertebral column are formed. Its appearance synchronizes with that of the neural tube. On the ventral aspect of the neural groove an axial thickening of the entoderm takes place; this thickening assumes the appearance of a furrow—the chordal furrow—the margins of which come into contact, and so convert it into a solid rod of cells—the notochord—which is then separated from the entoderm. It extends throughout the entire length of the future vertebral
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medial-ward and surrounds it. From the mesoderm surrounding the neural tube and notochord, the skull and vertebral column, and the membranes of the brain and medulla spinalis are developed.
THE PRIMITIVE SEGMENTS.
Toward the end of the second week transverse segmentation of the paraxial mesoderm begins, and it is converted into a series of well-defined, more or less cubical masses, the primitive segments (Figs. 18, 19, 20), which occupy the entire length of the trunk on either side of the middle line from the occipital region of the head. Each segment contains a central cavity—myocœl—which, however, is soon filled with angular and spindle-shaped cells.
The primitive segments lie immediately under the ectoderm on the lateral aspect of the neural tube and notochord, and are connected to the lateral mesoderm by the intermediate cell-mass. Those of the trunk may
