![]() Pontine flexure future cerebral hemispheres all 16 neuromeres presentįive subdivisions: medulla, pons, midbrain, diencephalons, telencephalon Figures 1–2B 1–3A, B 1–4 ![]() Mesencephalic flexure rhombencephalon, mesencephalon, prosencephalon 1–3 Sįusion of neural folds begins telencephalon and diencephalon distinguishable optic primordia 4–12 SĬaudal neuropore closes secondary neurulation begins 21–29 SĬlosed neural tube primordium of cerebellum isthmus rhombencephali 30–? Figure 1–2A Prominent features during the fetal period are the C-shaped structures, including the corpus callosum, and the appearance of sulci and gyri on the cortical surface at the middle of prenatal life. The appearance of the cortical plate (stage 21) heralds the beginning of lamination of the cerebral cortex, the basal nuclei and internal capsule are progressing, and the brain is developmentally advanced at the end of the embryonic period. The telencephalon is identifiable already at 4 weeks (stage 10) and begins to diverticulate at 5 weeks (during stage 14), but holoprosencephaly is more than a mere failure of diverticulation and may begin as early as 3 weeks (stage 8), as can cyclopia and anencephaly. The three major divisions of the brain are found early (stage 9), closure of the neuropores seals the cerebrospinal cavity at 4½ weeks (stage 13), and the five main subdivisions of the brain are visible at 5 weeks (stage 15). The embryonic period (the first 8 postfertilizational weeks) is subdivided into 23 morphological stages, which, because they are based on internal as well as external criteria, cannot be identified with confidence by ultrasonography. ![]()
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