Page:Popular Science Monthly Volume 24.djvu/597

Rh make their appearance. It undoubtedly contains a larger proportion of the nitrogenous matter which enters into the composition of all protoplasm, and, like the nuclei of other cells, a certain percentage of phosphorus. At this stage of its existence the germ (still microscopic) is represented in the zoölogical scale by the Amoeba, which it closely resembles in structure, having thus ascended to the second round of the zoölogical ladder.

The amœba has received its full share of attention from biologists. Its physiological endowments are scarcely greater than those of the non-nucleated moner. Both are capable of effecting those exchanges of matter which constitute nutrition; both are capable of reproduction (a phase of nutrition); both have the power of changing their form by thrusting out portions of their mass (the so called "false-feet"), and of thus executing slight creeping movements. These little masses of protoplasm are also capable of responding to contact of other matter, thus exhibiting the rudiments of common sensation. What is the evidence of this capacity? How does the amoeba manifest a sense of touch? When some substance, perhaps a smaller representative of its own species, floats against the surface of an amœba, the precocious bit of protoplasm responds to the salute by flowing around its victim, which is thereby inclosed within the body of its captor, and gradually appropriated as food. Probably the term "victim" is of doubtful application in this case, since the difference between eating and being eaten must be trifling. However that may be, the one improvises a stomach for the occasion, and digests the other with all the nonchalance of a Feejee-Islander. The human germ is, however, preserved from a similar indulgence in incipient cannibalism by its different environment—not the only period of its existence when it escapes evil-doing through lack of opportunity—for it receives its pabulum, ready prepared, from the blood of the mother, which is doubtless one of the conditions of its future higher development.

In this response to contact by movement on the part of the amœba, it exhibits the rudiments of both muscular and nervous action, since, under the influence of an external force or stimulus, a reflex movement is produced.

The next perceptible change in the evolution of the ovum is known as segmentation. This consists in an increase of its mass by duplication and reduplication; the single cell first acquires a second nucleus, and the surrounding protoplasm then separates into two masses, each having its own nucleus; this process is continued until the enveloping membrane contains a mass of cells, each like the original amœboid cell. From the resemblance of the ovum at this period to a mulberry, this is called the mulberry or morula stage of embryonic development. In the zoölogical scale, it corresponds to the labyrinthida, a little animal which consists of an aggregation of simple nucleated cells. From this multiplication of nuclei, which are regarded as the active centers