[The lateral geniculate body of the guinea pig (Cavia aperea f. domestica). Its organization with reference to retinal afferents].

The lateral geniculate nucleus (LGN) of the guinea pig was studied in order to investigate the existence of an alpha- and beta-sector. On the basis of the fibre arrangement and the cell distribution in its rostral third the LGd can be subdivided into an alpha- and beta-sector. The caudal half of the LGd comprises the alpha-sector only. In contrast to the alpha-sector, in which the cells are densely packed, the beta-sector is rich in fibre bundles and contains a scattering of medium-sized cells. Following unilateral enucleation we observed degenerating fibres in the contralateral beta-sector, too. This confirms the assumption that this sector is a part of the dorsal LGN. Like most of the alpha-sector, the beta-sector receives fibres from the contralateral retina, therefore the two sectors can not be interpreted like laminae. A cytoarchitectonical lamination could not be observed. The ipsilateral input is concentrated in a specific zone of the GdL (dorsomedial) but there is an overlapping with the contralateral input in the alpha-sector. In contrast to rat and mouse were the fibres of the Radiatio thalami are bordering the medial region of LGN the fibre bundles of the Radiatio thalami in the guinea pig and in the rabbit are scattered and pass through the medial part of the LGd. Therefore a beta-sector is formed. Parallel observations are reported from different species of marsupials. More investigations are necessary in order to solve the question whether the alpha- and beta-sectors--firstly described by Rose (1935) in the LGd of the rabbit--in lagomorphs, rodents and marsupials are homologous.