That the nucleus is the formative center of the cell is indicated by the following, among many facts: If a unicellular animal, such as =Stylonychia= (Fig. 2), for instance, be broken up into several fragments, it will be observed that some of the fragments are nucleated and others non-nucleated. The nucleated fragments have the power of quickly healing the wounds on them, regenerating the missing parts, and thus restoring the mutilated fragments to perfect individuals. These nucleated fragments have the power to perform all the activities of the perfect animal. The non-nucleated portions, on the other hand, cannot undergo regeneration. They cannot digest food, or grow or secrete substances as the nucleated fragments can. They can simply live for awhile, responding to stimuli and moving about. They finally perish. Having mentioned in a general way some of the wonderful powers of cells, it will be well now to describe briefly a few of the _unicellular plants and animals_ that can be so easily obtained and studied in warm weather, and which may thus serve as illustrations of the powers of nucleated pieces of protoplasm or cells. Many unicellular plants and animals can be obtained in summer from the superficial ooze on the bottom of slow-running streams and also on the under surfaces of the leaves of water plants, a study of which will be of the greatest value and interest. =Amœba proteus= (Fig. 3). This little unicellular animal, which belongs to the Rhizopod type, is very common in ponds and streams in warm weather. In the resting state it is spherical in form, but when active its form is as changeable as the fabled =Proteus=, hence its name, _Amœba proteus_. This little creature is a naked piece of protoplasm, with its outer layer differentiated into a firmer and pellucid part called the _ectoplasm_ (Fig. 3, ec); its interior, the _endoplasm_ (en), is quite granular and much more fluid, the granular particles moving quite freely upon one another when the animal changes its shape. The superficial portion of the endoplasm is firmer than its more central parts, and graduates insensibly into the more consistent ectoplasm. [Illustration: FIG. 3.—Amœba proteus: n, nucleus; cv, contractile vesicle; ec, ectoplasm; en, endoplasm; p, pseudopodia.]