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  1. 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.

  2. 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).

  3. 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.

  4. 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.

  5. 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.

Prenatal life can be divided conveniently into (1) the embryonic period proper, that is, the first 8 weeks following fertilization, and (2) the fetal period, which extends to birth. The distinction between the embryonic and fetal periods is well founded and has long been established in human embryology. The embryonic period is that time during which new features appear with great rapidity, whereas the fetal period is characterized more by the elaboration of existing structures. Moreover, the vast majority of congenital anomalies appear during the embryonic period. The difference is highlighted by the fact that the embryonic period has been successfully subdivided into morphological stages, whereas the fetal period has so far defied such a procedure.

The embryonic period has been divided into 23 developmental (Carnegie) stages (Table 2–1), which have been listed in detail by O’Rahilly and Müller,1 in whose monograph the early development of the human embryo has been thoroughly described. Each stage, on average, lasts slightly more than 2 days. The stages are based on both external and internal morphological criteria and depend mainly on features that change rapidly, such as the number of somitic pairs, the early appearance of the eye, and the form of the developing limbs. Although it may sometimes be possible to estimate approximately a given stage on ultrasonography, the staging system is based on having an embryo “in the hand” rather than in utero. Moreover, very early as well as late stages can be identified precisely only by histologic examination.


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