Nuclear configuration of the diencephalon of Tamandua tetradactyla and Myrmecophaga jubata

The nuclear configuration of the diencephalon of Tamandua tetradactyla (lesser anteater) is figured, described and compared with that of Myrmecophaga jubata (greater anteater) and other mammals, particularly the cat. Their nuclear organization represents, in general, all the elements (or their homologues) found in the mammalian diencephalon. There is an unusually large nucleus centralis lateralis which penetrates into the nucleus medialis dorsalis from above. The size of the latter group, in the presence of a poorly developed orbitofrontal cortex, suggests that it is primarily dependent upon the marked development of the corpus striatum. The nuclei of the tractus habenulopeduncularis are displaced ventrally, apparently by the medial enlargement of the nucleus centralis lateralis. The nuclei parataeniales bear strong resemblance to those in the armadillo. There is no nucleus ventralis pars anterior in either Tamandua or Myrmecophaga . The nucleus ventralis medialis is much larger than that in carnivores. The presence of the nucleus submedius and its intimate relation to the nucleus anterior medialis of the anterior nuclear complex, accounts fairly well for questionable homologues in such forms as Tarsius . The existence of both the large‐celled nucleus of the optic tract and the nucleus limitans indicates the gradual incorporation, at least in part, of the former into the latter. Hypothalamic nuclei, except for the supraoptic and paraventricular groups, are rather poorly differentiated.