Neural development in metatherian and eutherian mammals: variation and constraint.

A model for predicting the timing of neurogenesis in mammals (Finlay and Darlington [1995] Science 268:1578-1584) is here extended to an additional five metatherian species and to a variety of other events in neural development. The timing of both the outgrowth of axonal processes and the establishment and segregation of connections proves to be as highly predictable as neurogenesis. 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 between the event scales of the two; for metatherians, later events are slowed relative to earlier events. Furthermore, in metatherians, the timing of developmental events is more variable than in eutherians. The slowing of late developmental events in metatherians is associated with their considerably longer time to weaning compared with eutherians.