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MONG the simpler organisms known to biologists, perhaps the most simple as well as the most common is that which has received the name of Amœba. There are many varieties of amœba, and probably many of the forms which have been described are, in reality, merely amœbiform phases in the lives of certain animals or plants; but they all possess the same general characters. Closely resembling the white corpuscles of vertebrate blood, they are wholly or almost wholly composed of undifferentiated protoplasm, in the midst of which lies a nucleus, though this is sometimes absent. In many a distinction may be observed between a more solid external layer, or ectosarc and a more fluid granular interior, or endosarc; but in others even this primary differentiation is wanting. By means of a continually occurring flux of its protoplasmic substance, the amœba is enabled from moment to moment not only to change its form, but also to shift its position. By flowing round the substances which it meets, it, in a way, swallows them; and, having digested and absorbed such parts as are suitable for food, ejects or rather flows away from the useless remnants. It thus lives, moves, eats, grows, and after a time dies, having been during its whole life hardly anything more than a minute lump of protoplasm. Hence to the physiologist it is of the greatest interest, since in its life the problems of physiology are reduced to their simplest forms.

Now, the study of an amœba, with the help of knowledge gained by the examination of more complex bodies, enables us to state that the undifferentiated protoplasm, of which its body is so largely composed possesses certain fundamental vital properties:

1. —There can be little doubt that the changes in the protoplasm of an amœba, which bring about its peculiar "amœboid" movements, are identical in their fundamental nature with those which, occurring in a muscle, cause a contraction; a muscular contraction is essentially a regular, an amœboid movement an irregular flow of protoplasm. The body of the amœba may therefore be said to be contractile.

2. —When any disturbance, such as contact with a foreign body, is brought to bear n the amœba at rest, movements result. These are not passive movements, the effects of the push or pull of the disturbing body, and therefore proportionate to the force employed to cause them, but active manifestations of the contractility of the protoplasm; that is to say, the disturbing cause or