ONTOGENY OF THE PROJECTION TRACTS AND COMMISSURAL FIBERS IN THE FOREBRAIN OF THE TAMMAR WALLABY (MACROPUS-EUGENII) - TIMING IN COMPARISON WITH OTHER MAMMALS

The sequence of appearance of major forebrain projection and commissur al fibre bundles in the tammar wallaby (Macropus eugenii) during devel opment was examined with the aid of silver and haematoxylin stained ma terial. At the time of birth (PO), the cerebral cortex is unformed, bu t two prominent fibre bundles are apparent in the forebrain: the media l forebrain bundle and the stria medullaris thalami. There is also an unidentified tract (possibly thalamostriate or striothalamic), which a ppears to be transient, in that it cannot be identified at P8. By P2 t he posterior commissure, fasciculus retroflexus and mammillothalamic t ract have appeared. Fibres of the fornix were first visible at P8. Cor tical projection fibres (internal and external capsular fibres) were f irst noted at P10 and the anterior commissure at P14. It was not until P18 that the cortical commissural bundle unique to diprotodontid meta therians, namely the fasciculus aberrans, was first seen. The hippocam pal commissure was seen to develop relatively late, at P35. The sequen ce and tempo of development of these tracts has been compared in metat herian and eutherian forebrains. The sequence is similar in the two gr oups of mammals with one exception: isocortical commissural connection s appear to develop considerably earlier in diprotodontid metatherians than in eutherians. With regard to the tempo of forebrain tract devel opment, mammals with r selection reproductive patterns (large litter s izes, many litters per reproductive lifetime, rapid development of off spring, e.g. polyprotodontid metatherians, rodents) appear to have for ebrain tract development occupying a relatively greater proportion of the period from conception to the attainment of behavioural autonomy t han do those animals with K selection reproductive patterns (few offsp ring per reproductive lifetime, relatively prolonged development of of fspring, e.g. diprotodontid metatherians, primates). This difference i s irrespective of whether a mammal is metatherian or eutherian, indepe ndent of encephalization, and probably reflects the greater time alloc ated to aspects of brain development occurring after initial tract for mation (elaboration of cortical and forebrain circuitry, dendritic tre e growth, synapse overproduction and elimination) among K selection ma mmals.