The sequence of appearance of major forebrain projection and commissural fibre bundles in the tammar wallaby (Macropus eugenii) during development was examined with the aid of silver and haematoxylin stained material. At the time of birth (PO), the cerebral cortex is unformed, but two prominent fibre bundles are apparent in the forebrain: the medial forebrain bundle and the stria medullaris thalami. There is also an unidentified tract (possibly thalamostriate or striothalamic), which appears to be transient, in that it cannot be identified at P8. By P2 the posterior commissure, fasciculus retroflexus and mammillothalamic tract have appeared. Fibres of the fornix were first visible at P8. Cortical projection fibres (internal and external capsular fibres) were first noted at P10 and the anterior commissure at P14. It was not until P18 that the cortical commissural bundle unique to diprotodontid metatherians, namely the fasciculus aberrans, was first seen. The hippocampal commissure was seen to develop relatively late, at P35. The sequence and tempo of development of these tracts has been compared in metatherian and eutherian forebrains. The sequence is similar in the two groups of mammals with one exception: isocortical commissural connections appear to develop considerably earlier in diprotodontid metatherians than in eutherians. With regard to the tempo of forebrain tract development, mammals with r selection reproductive patterns (large litter sizes, many litters per reproductive lifetime, rapid development of offspring, e.g. polyprotodontid metatherians, rodents) appear to have forebrain tract development occupying a relatively greater proportion of the period from conception to the attainment of behavioural autonomy than do those animals with K selection reproductive patterns (few offspring per reproductive lifetime, relatively prolonged development of offspring, e.g. diprotodontid metatherians, primates). This difference is irrespective of whether a mammal is metatherian or eutherian, independent of encephalization, and probably reflects the greater time allocated to aspects of brain development occurring after initial tract formation (elaboration of cortical and forebrain circuitry, dendritic tree growth, synapse overproduction and elimination) among K selection mammals.
