The longitudinal growth of the neuromeres and the resulting brain in the human embryo.
The growth of the human brain during the embryonic period was assessed in terms of longitudinal measurements in staged embryos. Precise graphic reconstructions prepared by the onerous point-plotting method were considered to be the most reliable, and 23 were examined in detail. A distinction is necessary between measurements of the brain (cerebral diameters) and those of the skull (osseous diameters), and also between those of the folded brain in situ, studied here, and the later relatively straightened brain. Longitudinal measurements were made of individual neuromeres and their successors in steps (neuromeric lengths). The sum of the neuromeric measurements at any given stage provides the total neuromeric length (TNL) of the folded brain in situ at that stage and it increases in keeping with the greatest length (GL) of the embryo. At stages 16-19, however, the neuromeric length of the brain may exceed the GL. From stage 20 onwards the body length increases more rapidly compared with the length of the brain. The most cephalic neuromere is the telencephalon medium, abbreviated T1 here. The cerebral hemispheres are derived from it, although they are not neuromeres. The hemispheres soon extend rostrally beyond the limit of T1 by an amount that is here designated T2, and that indicates the growth of the telencephalon rostral to the commissural plate, which is the site of the future corpus callosum. Further laterally, the hemispheric length (future fronto-occipital diameter) increases rapidly, as does also the bitemporal (biparietal) diameter. At the end of the embryonic period these diameters are one fourth to one fifth of the head circumference. Additional neuromeric information becomes manifest when the measurements are calculated as percentages of the total length of the brain. The rhombencephalon decreases considerably, diencephalon 2 increases greatly, whereas diencephalon 1 diminishes, and the cerebral hemispheres enlarge massively. In addition, specific neuromeres or subdivisions come to occupy relatively more or relatively less of the total. Three periods were found during which individual neuromeres acquire their maximal or minimal lengths: the maximal absolute lengths were in period 3, whereas the maximal and minimal percentage lengths were in periods 1 and 3. The various neuromeric changes are considered to be related to alterations in functional development. Finally, in furtherance of establishing continuity in prenatal data, comparisons were effected between embryonic and fetal measurements.