Perhaps the most remarkable class of stimulations, however, is that due to the presence of the entire protoplasmic body of one organism in the cell of another, each living its own life for the time being, but the protoplasm of the host cell showing clearly, by its abnormal behaviour, that the presence of the foreign protoplasm is affecting its physiology. A simple example is afforded by Zopfs' _Pleotrachelus_, the amoeboid protoplasmic body of which lives in the hypha of _Pilobolus_, causing it to swell up like an inflated bladder, in which the parasite then forms its sporangia. The _Pleotrachelus_ does not kill the _Pilobolus_, but that its protoplasm alters the metabolic physiology of the latter is shown by the hypertrophy of the cells, and by the curious fact that it stimulates the _Pilobolus_ to form its sexual conjugating cells, otherwise rare, an indication of very far-reaching interference with the life-actions of the host. An equally remarkable example is that of _Plasmodiophora_, the amoeboid naked protoplasm of which lives and creeps about in the protoplasm of a cell of the root of a turnip, to which it gains access through the root-hairs. It does not kill the cell, but stimulates its protoplasm to increased activity and growth and division, itself dividing also and passing new amoebae into each new daughter-cell of the host. Here the processes of stimulation, hypertrophy and further division are repeated, until hundreds or thousands of the turnip root-cells are infected. The externally visible result is the formation of distorted swellings on the root (Finger and Toe), most of the cells of which are abnormally large and filled with amoeboid _Plasmodiophora_ protoplasm, which finally devours the turnip-protoplasm and itself passes over into spores. Here we have most convincing proof of the stimulation of protoplasm by other protoplasm in direct contact with it; and that the metabolism of the host-cells is profoundly altered is shown not only by the abnormal growth of the cells, but also by the starvation of the rest of the turnip plant as the _Plasmodiophora_ gets the upper hand. We have here, in fact, a local intracellular parasitic disease, gradually invading large tracts of tissue and eventually inducing general disease resulting in death--a state of affairs reminding us of cancer in animals.