Fig. 3. Aberrant projections of the corpus callosum and hippocampal commissure in COUP-TFI^null brains. Coronal sections through the telencephalon of E18.5 wild-type and COUP-TFI^null fetuses in which DiA was implanted in the rostromedial neocortex (A,A') and DiI in a more caudomedial position (B,B'). The inset in A indicates the positions of the dyes. Note the presence of Probst bundles (arrowheads in A',B') in the ipsilateral cortex. (D-E') Consecutive rostrocaudal sections, in which DiI was inserted in the hippocampus, show how the hippocampal commissure fails to cross in null mutants (arrows in D',E'). At all levels, labelled fibres do not cross the midline but turn ventrally. The white arrowheads in D and E indicate some retrogradely labelled neurons in the medial septum/diagonal band complex. Scale bars: 200 µm. NULL, COUP-TFI^null fetuses; Pb, Probst bundles; WT, wild-type fetuses.