The first appearance of the human nervous system at stage 8.

Eleven embryos of stage 8 (18 days), including a set of twins, were studied in detail and graphic reconstructions were prepared. This is the first report based on more than two specimens, as well as the first account of embryos of excellent histological quality at this developmental stage. Detailed measurements of the embryos are provided. It is found that the neural groove, which is the first morphological manifestation of the nervous system, is present in one quarter of embryos of stage 8. The neural folds are very largely cerebral rather than spinal. Correlations with other developmental features suggest that the neural groove is seen only when a certain degree of size and maturity has been reached by embryos of this important stage.