Neural development in metatherian and eutherian mammals: Variation and constraint

A model for predicting the timing of neurogenesis in mammals (Finlay and Da rlington [1995] Science 268:1578-1584) is here extended to an additional fi ve metatherian species and to a variety of other events in neural developme nt. The timing of both the outgrowth of axonal processes and the establishm ent and segregation of connections proves to be as highly predictable as ne urogenesis. Expressed on a logarithmic scale, late developmental events are as predictable as early ones. The fundamental order of events is the same in eutherian and metatherian animals, but there is a curvilinear relation b etween the event scales of the two; for metatherians, later events are slow ed relative to earlier events. Furthermore, in metatherians, the timing of developmental events is more variable than in eutherians. The slowing of la te developmental events in metatherians is associated with their considerab ly longer time to weaning compared with eutherians. (C) 1999 Wiley-Liss, In c.