The telencephalic vesicles are paired evaginations of approaches, particularly in mice and zebrafish, are rapthe anterior forebrain that constitute the most complex idly identifying genes that have crucial roles in forebrain and divergent structures in the vertebrate CNS. The prindevelopment. These studies have started to unravel the cipal components of the telencephalon are the pallium genetic pathways that pattern the telencephalon and (primarily the cerebral cortex in mammals) and the subare also proving to be instrumental in reassessing the pallium (which constitutes most of the basal ganglia). extent to which anatomical characteristics of the telen-Other telencephalic components, such as the septum cephalon are conserved or divergent in different classes and amygdala, have both pallial and subpallial origins. of vertebrate. Telencephalic function is integrally dependent upon its Here we review recent studies that have revealed tisconnections with other neural structures including the sue interactions and signaling pathways involved in the thalamus, hypothalamus, olfactory epithelium, and brainearly induction of the telencephalon and its subsequent stem. Together, these structures are essential for prosubdivision into dorsal and ventral territories. We begin cessing sensory information, integration of new sensory by describing genes that affect induction of the teleninformation with established memories (both experiencephalon, where new information has largely been tial and instinctual), and then formulating and effecting gained through genetic screens in zebrafish and gainbehavioral responses. Thus, in humans, the telencephaof-function assays in amphibia. We finish by describing lon is the seat of consciousness, higher cognition, lanprogress in our understanding of regional patterning of guage, motor control, and emotions; damage to this the telencephalon, where many of the advances have structure leads to dementia, specific sensory and motor come from targeted mutagenesis experiments in mice. deficits, language and movement disorders, and changes These studies have raised the intriguing possibility that in personality and emotional state. Although the telenearly regional patterning may be linked, through the excephalon probably controls related functions in all vertepression of particular transcription factors, to the specibrates, neuroanatomists have yet to reach a consensus fication of neuron-specific properties such as neuroof opinion regarding the homologies between mature transmitter phenotype. telencephalic nuclei and compartments in different classes of vertebrates. However, despite the highly vari-Induction of the Telencephalon able morphologies of the adult telencephalon, it is now The telencephalon derives from cells at the rostral marclear that the basic organization of telencephalic subdigin of the neural plate (Rubenstein et al., 1998; Varga visions is conserved during embryogenesis in all verteet al., 1999; Inoue et al., 2000; Whitlock and Westerfield, brates. 2000). While minor differences are apparent in fate maps