PLATE 15.
Embryonal Area.

FIG. 1.-Section through the Medullary Groove. The medullary groove (yellow, m) is formed from the ectoderm; laterodorsally are the two primitive ganglionated cords, from the cells of which the intervertebral ganglia are developed. Mesoderm, blue; chorda dorsalis, brown; entoderm and ectoderm, red.

FIG. 2.—Section through the Embryonal Area of a Three-day Chick Embryo. The medullary canal (m) is closed, forming in the middle the central canal, to either side of which are the prevertebral plates. Chorda dorsalis (ch), on either side the primitive blood-vessels (v), urinary tracts, etc.

FIG. 3.-Section through the Medullary Canal (First Month). The stratified epithelial cells which compose the wall are differentiated into spongioblasts (supporting substance) and neuroblasts (nervous substance). On the ventral side (r) the anterior roots (ra) are beginning to grow from the neuroblasts, which form the primitive anterior horns; on the dorsal aspect the sensory posterior roots (r.p) pass out from the cells of the spinal ganglia (g.i) into the two dorsal segments of the spinal cord.

FIG. 4.-Section through the Spinal Cord from the Second Month. The central gray matter is better supplied with nuclei and is beginning to be differentiated from the peripheral white substance; the anterior (ca) and posterior (ep) horns are formed; roots as in figure 3; central canal (c).

FIG. 5.-Section through the Spinal Cord from the Eighth Month. In the white substance the various systems of nerve-fibers successively. undergo the process of medullation, so that nerve tracts anatomically and functionally related reach their full development at the same time. The pyramidal tract (Py.l and a) and certain portions of the posterior columns (g) are the last to become medullated. The details of the process can be studied in the figure, as the fibers already covered with the medullary sheath are stained black, while the nonmedullated portions are unstained and appear brownish in color.

FIG. 6.-Medulla Oblongata of a Six-weeks Embryo. Lamina fundamentalis (f); lamina ependymica (le); lamina tegmenti (7W); VIII, XII, acusticus, hypoglossus; Py, pyramids; r. IV, fourth ventricle.

PLATE 16.

Development of the Brain.

FIG. 1.-Brain and Cord from the Third Month. The forebrain (I) is already divided into its hemispheres, and covers the interbrain and midbrain (III) (primitive optic thalami and corpora quadrigemina). Posteriorly the hindbrain (IV) (primitive cerebellum) and afterbrain (V) (primitive medulla). The cord shows the position of the two enlargements (J. C and J.L).

FIG. 2.-Base of the Brain (Fourth Month). The frontal lobe is divided from the temporal lobe (L.f., Lt) by the fissure of Sylvius (F.S); the chiasm (II) and the pes (Pe) are distinguishable. Cerebellum (C.b), pyramids (Py), olives (0).

FIG. 3.-Mesial Surface of the Brain (Fourth Month). The corpus callosum (c.e) and fornix (ƒ) are readily recognized; in front and between the two is the septum lucidum (8.p). The descending limb of the fornix is reflected downward and backward and ends in the corpus albicans.

FIG. 4.-Outer Surface of the Cerebral Hemispheres (Sixth Month). The upper surface shows a few fissures. The fissure of Rolando grows from above downward, the fissure of Sylvius (f.S) from below upward. At the bottom of the fissure of Sylvius the primitive island (J). C'b, cerebellum; P, pons; Py, pyramid; o, olive; Mo, medulla oblongata.

FIG. 5.-Mesial Section through the Primitive Brain of a Two and One-half Months' Embryo. The roof of the primitive ventricular cavity is formed by: Lamina tegmenti (t); cerebellum (Cb); primitive corpora quadrigemina (qp, qa); epiphysis (E); tænia thalami (th); pallium of the cerebral vesicle (I). The floor is formed by the bulb of the medulla (Mo); pons (P); corpus albicans (e); hypophysis (H); and in front of the latter, the lamina embryonalis. The optic thalamus (Th), foramen of Monro, subthalamic region, and olfactory bulb are shown.