Other as simple but larger organisms may consist of a minute mass of semi-fluid protoplasm, containing granules, as we find to be the case in the plant _Vaucheria_,[85] and many other _Algæ_, and in the animal _Amœba primitiva_.[86] An organism of this simplest kind or a fragment of a higher organism which presents this simplest condition is called a cell.[87] Very generally such cell has within it a more or less distinctly marked generally denser and spheroidal body called a _nucleus_, within which, again, other minute spots may appear called _nucleoli_. Even in this simplest of all possible conditions of life a slight difference appears between its most external film and its inner substance--just as a cup of broth left to stand will form for itself a filmy outermost layer. This incipient difference between what is inner and what is outer is one which is constantly maintained in all higher organisms, as we shall soon see abundantly. But the distinction into outer and inner is, as has been said, shown in a much more marked way in the constituent units, or _cells_, which build up the bodies of plants generally; for these consist of an inner part of protoplasm, enclosed in a distinct external cellulose envelope or _cell-wall_. As has also been shown, many of the lowest animals take on occasionally the _encysted_ condition when they also consist of a particle of bioplasm enclosed in a distinct cell-wall or _cyst_, though one not made of cellulose. The protoplasmic contents of the cell may attract watery fluid thus forming clearer spaces or _vacuoles_ within it, and these may become so extended that the protoplasm may be reduced to a thin layer lining the cell wall, thread-like processes or remnants of protoplasm often passing across the cell from one part of the protoplasmic lining to another. A cell, almost always a nucleated cell, is the original form of every living creature without exception; and a great number of small, and some considerably sized living beings, never get beyond this unicellular condition, however much their cell may become enlarged or complicated in shape. Such creatures form the lowest of all animals and plants; but the overwhelming majority of living creatures are formed of aggregations of cells which cohere and fuse together in various ways. As an example of a unicellular and typically cellular living creature we may take the yeast plant (_Saccharomyces cerevisiæ_), which consists of a particle of bioplasm enclosed in a cell-wall of cellulose, the whole being globular or oval in shape, and generally about 1/3000 of an inch in diameter. Within its bioplasm a clear space or vacuole may often be distinguished. Often these organisms appear with a more complicated outline, due to the growth of new saccharomycetes from its outer wall, and the budding forth of others again from the side of such protruding processes, all of which ultimately become detached as independent saccharomycetes, though they