the gamma-aminobutyric acid ( gaba ) system has been found to differentiate very early relative to other transmitter-identified neurons , such that by embryonic day 13 a well developed fiber network exists in the brainstem , mesencephalon and diencephalon , including a large projection in the posterior commissure and adjacent areas on the surface of the mesencephalon and tectum .
C0025462
mesencephalon
C0039433
tectum