VI. The most important step in the gradual construction of the reflex mechanism is the division into three cells; in the place of the simple connecting bridge we spoke of there appears a third independent cell, the _soul-cell_, or ganglionic cell; with it appears also a new psychic function, _unconscious presentation_, which has its seat in this cell. The stimulus is first conducted from the sensitive cell to this intermediate presentative or psychic cell, and then issued from this to the motor muscular cell as a mandate of movement. These _tricellular reflex organs_ are preponderantly developed in the great majority of the invertebrates. VII. Instead of this arrangement we find in most of the vertebrates a _quadricellular reflex organ_, two distinct "soul-cells," instead of one, being inserted between the sensitive cell and the motor cell. The external stimulus, in this case, is first conducted centripetally to the sensitive cell (the sensible psychic cell), from this to the _will-cell_ (the motor psychic cell), and from this, finally, to the contractile muscular cell. When many such reflex organs combine and new psychic cells are interposed we have the intricate reflex mechanism of man and the higher vertebrates. The important distinction which we make, in morphology and physiology, between unicellular and multicellular organisms holds good for their elementary psychic activity, reflex action. In the unicellular protists (both the plasmodomous primitive plants, or _protophyta_, and the plasmophagous primitive animals, or _protozoa_) the whole physical process of reflex action takes place in the protoplasm of one single cell; their "cell-soul" seems to be a unifying function of the psychoplasm of which the various phases only begin to be seen separately when the differentiation of special organs sets in. The second stage of psychic activity, compound reflex action, begins with the cenobitic protists (_v.g._, the volvox and the carchesium). The innumerable social cells, which make up this cell-community or coenobium, are always more or less connected, often directly connected by filamentous bridges of protoplasm. A stimulus that alights on one or more cells of the community is communicated to the rest by means of the connecting fibres, and may produce a general contraction. This connection is found, also, in the tissues of the multicellular animals and plants. It was erroneously believed at one time that the cells of vegetal tissue were completely isolated from each other, but we have now discovered fine filaments of protoplasm throughout, which penetrate the thick membranes of the cells, and maintain a material and psychological communication between their living plasmic contents. That is the explanation of the mimosa: when the tread of the passer-by shakes the root of the plant, the stimulus is immediately conveyed to all the cells, and causes a general contraction of its tender leaves and a drooping of the stems.