It has already been stated that the most active cells, such as are found in the earliest stages in the life of an organism (embryonic cells), have no inclosing membrane or cell wall. But in the more advanced stages of cell life we find this second form of protoplasmic differentiation to be common enough. In animal cells the limiting membrane has never the same importance as the cell wall in vegetable tissues, where some of the principal textures may be traced to a direct modification of the cell wall, still recognizable as such. Whenever such a limiting membrane exists, it is formed by the outer layers of protoplasm undergoing changes so as to become of greater consistence. In the animal tissues the cells form various structures, which are not limiting membranes or cell walls, but rather give the idea of lying between the cells. Hence, in one large group of tissues, they have been called intercellular substance, while in others they appear as materials specially modified for the furtherance of the functions of the special tissues.