The human brain at stages 21-23, with particular reference to the cerebral cortical plate and to the development of the cerebellum.

The development of the human brain during the eighth embryonic week was studied in serial sections of 22 embryos, and graphic reconstructions were prepared. The cortical plate appears in stage 21 in the area of the future insula and is an excellent feature for staging. The internal capsule contains neocortical fibres. Its three main outlets begin to be present in stage 22 and lead to epithalamus, to dorsal thalamus, and to mesencephalon. At this time a well developed lateral olfactory tract can be seen. The anterior commissure appears in stage 23. A clear developmental relationship between claustrum and olfactory area is described for the first time in human embryos. The optic tract reaches the ventral area of the lateral geniculate body. Scattered fibres of the lateral lemniscus reach at least as far as the caudal mesencephalon, in which superior and inferior colliculi can be distinguished at stage 23; two caudal Blindsäcke containing ventricular recesses form in stage 23. The cerebellum is still present as a plate, but its internal bulge is considerably enlarged. It possesses radially- and tangentially-arranged cells; the latter form the external germinal layer. The dentate nucleus, as well as the inferior and superior cerebellar peduncles and some of the cerebellar commissures, are present. Compared with the highly developed and probably already functional remainder of the hindbrain, the cerebellar plate shows far less differentiation. Two caudal migratory streams (marginal and submarginal) are present and represent the corpus pontobulbare. The decussation of the pyramids appears in stage 23. This article concludes the study of the developing human brain during the embryonic period, from stage 8 to stage 23. The series was based on 340 serially-sectioned embryos and graphic reconstructions from 89 brains. No comparable investigation of the fetal brain is available.