Page:Popular Science Monthly Volume 15.djvu/748

728 colored food, which will then be seen to have accumulated in the interior of their soft, transparent protoplasm; and in some cases the colorless blood-corpuscles have actually been seen to devour their more diminutive companions, the red ones.

Again, there are certain cells filled with peculiar colored matters, and called pigment-cells, which are especially abundant, as constituents of the skin, in fishes, frogs, and other low vertebrate, as well as many invertebrate, animals. Under certain stimuli, such as that of light, or of emotion, these pigment-cells change their form, protrude or retract pseudopodial prolongations of their protoplasm, and assume the form of stars or of irregularly lobed figures, or again draw themselves together into little globular masses. To this change of form in the pigment-cell the rapid change of color, so frequently noticed in the animals provided with them, is to be attributed.

The animal egg, which in its young state forms an element in the structure of the parent organism, possesses in the relations now under consideration a peculiar interest. The egg is a true cell, consisting essentially of a lump of protoplasm inclosing a nucleus, and having a nucleolus included in the interior of the nucleus. While still very young it has no constant form, and is perpetually changing its shape. Indeed, it is often impossible to distinguish it from the Amœba; and it may, like an Amœba, wander from place to place by the aid of its pseudopodial projections. I have shown elsewhere that the primitive egg of the remarkable hydroid Myriothela manifests amœboid motions; while Haeckel has shown that in the sponges certain Amœba-like organisms, which are seen wandering about in the various canals and cavities of their bodies, and had been until lately regarded as parasites which had gained access from without, are really the eggs of the sponge; and a similar amŒboid condition is presented by the very young eggs of even the highest animals.

Again, Reichenbach has proved that during the development of the crawfish the cells of the embryo throw out pseudopodia by which, exactly as in an Amœba, the yolk-spheres, which serve as nutriment for the embryo, are surrounded and ingulfed in the protoplasm of the cells.

I had shown some years ago that in Myriothela pseudopodial processes are being constantly projected from the walls of the alimentary canal into its cavity. They appear as direct extensions of a layer of clear, soft, homogeneous protoplasm, which lies over the surface of the naked cells lining the cavity, and which I now regard as the