. Comparative anatomy. Anatomy, Comparative. = -" . 1a"l>r pl>te ibasal plate ifloor plate - ' MESEINCEPHALON EPIPHYSIS TELENCEPHALON METENCEPHALON MYELENCEPHALON PREOPTIC RECESS Fig. 447.—Diagrams of the development of the brain. A, Early neural plate before closure, with zones marked; B, Longitudinal section of early brain tube; C, Later stage with parts diiTerentiated. The dorsal zone (alar plate) is finely stippled; the ventral zone (basal plate) is coarsely stippled; the floor plate is cross-hatched. Cer., cere- bellum; , corpus striatum; inf., infundibu
. Comparative anatomy. Anatomy, Comparative. = -" . 1a"l>r pl>te ibasal plate ifloor plate - ' MESEINCEPHALON EPIPHYSIS TELENCEPHALON METENCEPHALON MYELENCEPHALON PREOPTIC RECESS Fig. 447.—Diagrams of the development of the brain. A, Early neural plate before closure, with zones marked; B, Longitudinal section of early brain tube; C, Later stage with parts diiTerentiated. The dorsal zone (alar plate) is finely stippled; the ventral zone (basal plate) is coarsely stippled; the floor plate is cross-hatched. Cer., cere- bellum; , corpus striatum; inf., infundibulum; mam., mammillary recess; olf., olfactory lobe; pal., pallium; thai., thalamus. (After Kingsbury modified.) Since this sulcus does not develop in the forebrain, and ends where the notochord ends, it appears that the forebrain consists of alar and roof plates only. By the time a human embryo is a month old, the primitive forebrain vesicle has begun to divide into the anterior telencephalon and posterior diencephalon. In a five weeks embryo, the hindbrain has begun to divide into the anterior metencephalon and posterior myelen- cephalon. The undivided midbrain is the mesencephalon. These five brain regions occur in all vertebrates, and from them all the parts of the adult brain are formed. Brain Flexures. While the subdivision of the primary vesicles is taking place in ontogenesis, the brain undergoes in amniote embryos three successive flexures, the cephalic or primary, the pontine, and nuchal or cervical. The cephalic flexure occurs in the midbrain region, the other two in the region of the hindbrain. All three flexures are in a vertical plane, but the bend of the pontine is the reverse of the other two. The. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940;
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