The _essential_ structures which may by appropriate means be distinguished in the bacterial cell are _cell wall_ and _cell contents_, technically termed _protoplasm_, cytoplasm. The cell wall is not so dense, relatively, as that of green plants, but is thicker than the outer covering of protozoa. It is very similar to the cell wall of other lower fungi. Diffusion takes place readily through it with very little selective action on substances absorbed as judged by the comparative composition of bacteria and their surrounding medium. =Cytoplasm.=--The cytoplasm according to Bütschli and others is somewhat different and slightly denser in its outer portion next to the cell wall. This layer is designated the _ectoplasm_, as distinguished from the remainder of the cell contents, the _endoplasm_. When bacteria are suddenly transferred from a given medium into one of decidedly _greater_ density, there sometimes results a contraction of the _endoplasm_, due to the rapid diffusion of water. This phenomenon is designated _plasmolysis_ (Fig. 17), and is similar to what occurs in the cells of higher plants when subjected to the same treatment. This is one of the methods which may be used to show the different parts of the cell just described. If bacteria are suddenly transferred from a relatively dense medium to one which is of decidedly _less_ density, it occasionally happens that water diffuses into the cell and swells up the endoplasm so much more rapidly than the cell wall that the latter ruptures and some of the endoplasm exudes in the form of droplets on the surface of the cell wall. This phenomenon is called _plasmoptysis_. Students will seldom observe the distinction between cell wall and cell contents, except that in examining living bacteria the outer portion appears more highly refractive. This is chiefly due to the presence of a cell wall, but is not a proof of its existence. [Illustration: FIG. 17.--Cells of bacteria showing plasmolysis. The cell substance of three of the cells in the middle of the chain has shrunk until it appears as a round black mass. The cell wall shows as the lighter area.] [Illustration: FIG. 18.--Vacuoles in the bacterial cell. The lighter areas are vacuoles.] =Nucleus.=--Douglas and Distaso[3] summarize the various opinions with regard to the nucleus in bacteria as follows: 1. Those who do not admit, the presence of a nucleus or of anything equivalent to it. (Fischer, Migula, Massart). 2. Those who consider that the entire bacterial cell is the equivalent of a nucleus and contains no protoplasm. (Ruzicka). 3. Those who admit the presence of nuclein but say that this is not morphologically differentiated from the protoplasm as a nucleus. (Weigert). 4. Those who consider the bacterial protoplasm to consist of a central endoplasm throughout which the nuclein is diffused and an external layer of ectoplasm next to the cell wall. (Bütschli, Zettnow).