Longitudinal callosal fascicles, or Probst bundles, are aberrant bundles of axons that run in a front-back (antero-posterior) direction rather than a left-right direction between the cerebral hemispheres. They are characteristic of patients with agenesis of the corpus callosum and are due to failure of the callosally-projecting neurons (mostly layer 2/3 pyramidal neurons) to extend axons across the midline and therefore form the corpus callosum. The inability of these axons to cross the midline results in anomalous axonal guidance and front-to-back projections within each hemisphere, rather than connecting between the hemispheres in the normal corpus callosum.
^Barkovich, AJ.; Norman, D. (Jul 1988), "Anomalies of the corpus callosum: correlation with further anomalies of the brain.", AJR Am J Roentgenol, 151 (1): 171–9, doi:10.2214/ajr.151.1.171, PMID3259802, S2CID15520310