most posterior part of the fly brain, the tritocerebrum, is posterior
to the boundary and expresses a homeotic gene, suggests that
the cephalized cord actually comprises four segments. In chor-
dates there is some doubt as to whether the intermediate region
comprises 8 segments, as usually considered, or 4 segments
each subdivided in two compartments, as many features suggest.
While it may be that this specialized region appeared indepen-
dently in arthropods and in chordates as a cephalization of the
anteriormost part of the segmented nerve chord, it would seem
more economical to imagine that urbilaterians already possessed
a four-tiered CNS with a brain proper (protobrain), a boundary
(MHB) region, a segmented hindbrain, and a nerve chord.