Afferent connections to the tectum mesencephali in the hagfish, Eptatretus burgeri: an HRP study

The afferent connections to the tectum mesencephali were determined by unilateral injections of HRP in the hagfish, Eptatretus burgeri, whose eyes are degenerate and buried below the skin. Retrogradely labeled cells were observed ipsilaterally in the hypothalamus, lateral part of the pars ventralis thalami of Jansen, and nucl. reticularis mesencephali, and contralaterally in the tectum, nucl. sensorius n. V, area acoustico-lateralis, and "nucl. funiculi dorsalis". Of these non-visual afferents to the tectum, the trigeminal sensory nucleus contributed the greatest number, which ascended along the lemniscus bulbaris after crossing in the commissura ventralis. Retrogradely labeled fibers of the optic nerve could not be recognized in this study, although in a previous study the superficial layer of the tectum mesencephali received a few projections from the contralateral eye. These data are compared with the data from other animals and previous histochemical investigations.