Comparative histology demonstrates the different ways through which a function can be performed by one or more structures. The functional approach offers the widest comparison, much wider than any morphologic comparison would be able to do. The function adds the physiological and biochemical approach to the picture. Histochemistry bridges the gap between both, the comparative and functional aspect, as well as tissue and organ culture. The tissues show a variation in their phylogenetic age of appearance and taxonomic distribution. Comparative functional histology is a basic science of the various biological disciplines. Comparative functional histology is also a basic pillar of experimental pathology. The functional frame of comparative histology has been outlined. The following communications of the series listed below will give a detailed description of special aspects of comparative functional histology.