Significant features in the early prenatal development of the human brain.

A review of the early prenatal development of the human brain has been prepared following a long-standing investigation of 192 embryos. The precise sequence of developmental events has been traced with the aid of accurate morphological staging. The three major divisions of the brain appear in the walls of the completely open neural groove at 3(1/2) postfertilizational weeks (stage 9). They do not develop as "cerebral vesicles" in a closed neural tube. The 16 neuromeres and the various subdivisions of the neural crest are highlighted. It is stressed that only two neuropores are normally found in the human. The telencephalon can be distinguished as early as 4 weeks (stage 10) and the five chief subdivisions of the brain are recognizable at 5 weeks (stage 15). The development of the medial (diencephalic) and lateral (telencephalic) ventricular eminences (so-called Ganglienhügel) is elaborated, and their role in the formation of the basal nuclei is clarified. The cortical plate and subplate have been identified as early as 7 weeks (stage 21). Finally, it is pointed out that the timing of the origin of many congenital anomalies of the nervous system shows the special importance of the embryonic period, i.e., the first 8 postfertilizational weeks.