The general properties of protoplasm may be readily observed in the simplest organic forms, like the Amæba, that are usually described as simple masses of protoplasm without structure or any distinction of parts. It should be remarked, however, that numerous species of Amæba have been described, differing in form and to some extent in habits, and there may also be differences in their protoplasm which we are unable to detect with our present means of investigation.
Under the low powers of the microscope an Amæba appears as a semi-transparent, jelly-like mass, which glides along with a flowing movement of its apparently homogeneous substance, sending out armlike projections from any part of its body to close around substances which it can feed upon, and rejecting other materials unsuitable for its nutrition. The processes of prehension, digestion, assimilation, respiration, excretion, and reproduction are carried on by the entire body, or by any part of it indifferently. The body of an Amæba, as we observe it, is not, however, a simple mass of protoplasm, as it evidently contains particles of undigested food, with particles representing the various stages the elements of the food pass through in being built up into protoplasm, together with the various waste products on the way to be excreted, so that what we call protoplasm, as represented in an Amæba, contains many extraneous substances; and substantially the same statement may also be made in regard to the differentiated protoplasm of the higher plants and animals.
From this it must be seen that it is practically impossible to obtain samples of pure protoplasm for analysis, and, even if this could be done, a chemical analysis of living protoplasm can not be made; but there is, however, evidence to show that there must be a wide difference in the chemical properties of living and of dead protoplasm. Carmine and other coloring matters, for example, do not color living protoplasm, but give a brilliant stain to dead protoplasm; and other observations show that living substance has properties that interfere with or limit the ordinary chemical and physical reactions of dead matter.
There are other considerations in regard to the composition of protoplasm which require a reference to the food of the higher animals, which is usually said to consist of the so-called proteids, fats, and carbohydrates, to which should be added certain mineral constituents or salts, with oxygen introduced by the lungs. These groups of food-stuffs have not the physiological significance that was formerly attached to them, and they do not represent definite chemical compounds which have a specific rôle in the processes of nutrition, as each group includes a great variety of complex compounds. The proteids or albuminoids, as they are sometimes called, are a group of organic substances containing carbon, hy-