Page:George Archdall Reid 1896 The present evolution of man.djvu/44

32 we may conveniently begin the study of life at that stage of evolution which they have reached. Watched under the microscope the amœba is known by its actions to be a living being. It puts forth or withdraws thinner or thicker processes known as pseudopodia. It moves by flowing forward, or by putting forth pseudo-podia and flowing into them. Streaming motions of granules may be observed in its substance. It engulfs food particles, and having assimilated the digestible portions, flows away, leaving the indigestible remainder behind. It shrinks from harmful contact.

In describing the amœba I have described also the white blood corpuscle, the leucocyte of man and other animals. These occur in great numbers, maintaining a separate existence in the blood, or wandering through the tissues. It might be thought that they are parasites, but this is disproved by the fact that if any tissue is injured, as for instance by a cut, they crowd in countless numbers to the spot, and repair the injury with their bodies, which thereupon undergo changes in shape and structure. What is known as pus or "matter," such as flows from an abscess, is a clear fluid rendered turbid by the multitude of their dead. In cases of zymotic disease they have been seen with the invading microbes enclosed in their substance, when either the leucocyte or the microbe perishes.

Both the amoeba and the leucocyte multiply by fission, by dividing the one into two. First the nucleus divides by a complicated process known as Karyokinesis, the completion of which is followed by the division of the cell body. The daughter-cells grow, and in time divide in the same manner as the parent. This process is repeated through an immense though finite number of generations by the leucocyte, and through an infinite number of generations by the amœba. Whence it is clear that the amœba is