continually from the interior of the vital system into the surrounding medium, so that on the periphery of the system it cannot always be said whether or not a component still belongs to life. Considering these circumstances we can roughly for the present define the conception of stimulus as follows: _A stimulus is every change in the vital conditions._ The most essential point in this definition is the relation of the conception of stimulus to that of vital conditions. These relations, however, call for a brief explanation. Here again the conditional method of observation saves us from error, for it would be wrong to place the conception of stimulus and vital conditions in contrast to one another, one excluding the other. On the other hand, this method of observation shows that the stimuli are likewise only conditions, but conditions producing certain changes in the vital system. If a stimulus acts, that is, if there is any change whatever in the vital conditions, the whole complex of life in consequence of the dependency of the constituent parts upon each other is also changed, and a new state of living substance occurs. Stimuli are, therefore, also only vital conditions, but vital conditions for new vital manifestations. The _relation_ of _one_ given state to _another_, forms an indispensable point in the understanding of vital conditions as well as that of the stimulus. The stimulus becomes a vital condition for the new state which it produces. It is only a stimulus _relatively_ to the original state, which _previously_ existed. The essential point, therefore, in the conception of the stimulus is that of alteration. An example will serve to make this clearer. If _Amœba limax_ are bred in a hay infusion they appear in countless masses. Observed in water in a watch glass they show at first the well-known form of _Amœba proteus_ with short, broad, lobate pseudopods. (Figure 1, A.) After a period of rest, however, they gradually assume the characteristic elongated _limax_ form. (Figure 1, B.) In this shape they constantly move about. But if I add to the water only a faint trace of diluted solution of caustic potash, the amœbæ first assume the shape of a ball (Figure 1, C), and then after a time, stretch out long, pointed pseudopods, which give them the characteristic form of _Amœba radiosa_. (Figure 1, D and E.) They remain permanently[16] in this form. I have observed them for several hours at a time. They move in the same manner as _Amœba radiosa_. They draw in one pseudopod, stretch out another and float freely in the water in contrast to their _limax_ state, in which they are always attached to some support. The long, pointed, often threadlike pseudopods, yield to every movement of the water, bending in consequence like whipcords. In this example the amœbæ under the vital conditions existing in tap water have _limax_ form. The vital conditions undergo a change by the addition of a solution of caustic potash, which acts as a stimulus.