1. Serial sections of the brains of man and 79 other mammals, 4 birds and 6 reptiles, i. e., 90 animals in total, were examined in this study.
2. The external medullar plate and the thalamic reticular stratum are not respectively separate formations but represent only different parts of extension of the thalamic radiation, wherein the radiation fibres change direction before entering the internal capsule and from the picture of reticular formation on the cross-sections; consequently, no such layers can be observed in the cases where the radiation fibres run nearly straight into the internal capsule.
3. The NRT was found absent in birds and reptiles, as well as in Monotremata, and appears in Marsupialia and higher orders of mammals alone. Thus, it is a phylogenetically very new formation.
4. The NRT extends never beyond the lateral thalamic nucleus and the pulvinar. NRT cells are found infiltrated into the external medullar plate, so that the NRT is connected through gray matter with the pulvinar and the lateral nucleus; the cells in these three formations are hard to differentiate morphologically. From these findings and the above-described fibre architecture, I am led to the conclusion that the NRT is in one system with the lateral nucleus and the pulvinar, but not entirely identical in nature with these, for it is the phylogenetically newest part of this system.
5. The relative volume of this nucleus is small in Rodentia and the lower orders but larger in Cetacea and the higher orders, but in these higher orders, no special trend of change in the relative volume could be established.
6. The NRT shows no regional specificity and must be deemed as one homogenous nucleus.
7. The NRT is concluded to belong to the so-called thalamic reticular system, and the so-called ascending reticular activating system is once interrupted in the parts of the thalamic reticular system outside the NRT, the diffuse thalamic projection system starting thence is relayed through the NRT and finally runs toward the cerebral cortex.