The first appearance of the major divisions of the human brain at stage 9.

Five embryos of stage 9 (20 days) were studied in detail and graphic reconstructions were prepared. This is the first report based on more than one specimen of this rarely seen stage. Detailed measurements of the embryos are provided. The characteristic features of this stage are 1 to 3 pairs of somites, the head fold and foregut, the otic discs, and the pericardial cavity. The primitive streak is not decreasing as rapidly as in stage 8 but the caudal eminence is proliferating. The notochordal plate is not increasing as rapidly as in stage 8 and a notochord is not yet present. It is to be stressed that the 3 major divisions of the brain (prosencephalic, mesencephalic, and rhombencephalic) can be identified in the open neural groove. The rhombencephalon is the dominant feature of the brain and comprises 4 subdivisions, the last of which, previously unrecognized, is related to the (occipital) somites and represents the hypoglossal region. Features that may not be visible include the caudal fold and hindgut, endocardial tubes or plexus, neural crest, and neurenteric canal. In 2 specimens, atria, left ventricle, right ventricle, conotruncus, and first (or first and second) aortic arches are distinguishable. Two of the embryos studied are the earliest examples in which neural crest has been identified.