Usually, however, a cell is small—just as much protoplasm as its still more minute nucleus can keep going; though here, again, one must be guarded, for there may be several nuclei instead of only one. The protoplasm on the external surface and around the nucleus is specialized into a more or less definite membrane. To this outer envelope are attached fine fibrils, which join up to a small body within the cell, called the centrosome, and by the lengthening and shortening of these its shape can be altered. The contents are fluids; so if the containing membrane is loosened in any direction, they tend to bulge out and form an excrescence, and in this way the cell is enabled to throw out limbs and surround particles of food, and, by relaxing the fibrils in one direction and contracting them in others, to crawl whither its chemical, thermal, or physical affinities direct. (See Diagram 1.) Not particularly inspiring is the sight of life in its simplest form, but when a few millions of these cells group together and form one body, dividing the labour between them, the result is something stupendous. There are animals composed in this way, some of whose cells have developed their digestive capabilities to such an extent that they have almost lost all their others. These are carefully guarded in the interior of the body. Other cells in this same beast, receiving their food in a fluid form from these digestive specialists, secrete lime around them till a skeleton is built up. To the levers of this skeleton are attached bundles and strands of cells, which, if they can do nothing else, can lengthen and shorten and make it move. Yet, again, there are cells which have especial facilities for receiving, weighing, and transmitting chemical and physical promptings. These cells, again, lie in a protected corner of the interior, but they send out fine threads to one another and every part of the body, and control the whole. The animal in which this beautiful system of division of labour has been carried to its greatest perfection has many and varied powers. He can in some cases even apply to the individuals of the species the principles of his own cellular economy, and thereby achieve not only the making of poetry and jokes, but the building of a Westminster Abbey, the construction of Maxim guns, and the enforcing of his economic refinements upon his less highly specialized neighbours.