part of start stage end stage diencephalic white matter cranial nerve II Cranial nerve which is comprised of retinal ganglion cell axons running posterior medially towards the optic chiasm, at which the axons cross the midline and after which the structure is termed the optic tract. zebrafish_anatomy optic TAO:0000435 optic nerve ZFA:0000435 CN-II cranial nerve hatching:long-pec adult