Henry Gray (18211865). Anatomy of the Human Body. 1918. 2.
Development of the Nervous System The entire nervous system is
of ectodermal origin, and its first rudiment is seen in the
neural groove which extends along the dorsal aspect of the
embryo (Fig. 17). By the elevation and ultimate fusion of the
neural folds, the groove is converted into the neural tube (Fig.
19). The anterior end of the neural tube becomes expanded to
form the three primary brain-vesicles; the cavity of the tube is
subsequently modified to form the ventricular cavities of the
brain, and the central canal of the medulla spinalis; from the
wall the nervous elements and the neuroglia of the brain and
medulla spinalis are developed. FIG. 640 Section of medulla
spinalis of a four weeks embryo. (His.) (See enlarged image)
FIG. 641 Transverse section of the medulla spinalis of a human
embryo at the beginning of the fourth week. The left edge of the
figure corresponds to the lining of the central canal. (His.)
(See enlarged image) The Medulla