As to the form in which the last six elements in the table exist in the cell, little is known. The sulphur and phosphorus are essential constituents of various proteins. The high percentage of phosphorus points to nuclein compounds as its probable source. The carbon and nitrogen, together with most of the hydrogen and oxygen not united as water, make up the great variety of organic compounds which compose the main substances in the bacterial cell. It has already been stated that the essential structures in the bacterial cell are cell wall and protoplasm, including the nuclein. These differ markedly in chemical composition. It is well known that the cell walls of green plants consist largely of cellulose and closely related substances.[7] _True cellulose_ has been recognized in but very few bacteria. (_Sarcina ventriculi_, Migula; _Mycobacterium tuberculosis_, Hammerschlag, Dreyfuss, Nishimura; _Bacillus subtilis_, Dreyfuss; _Acetobacter xylinum_, Brown; _Acetobacter acidi oxalici_, Banning; and a few others.) It is certainly not an important constituent of the cell wall in many. On the other hand, _hemicellulose_ and _gum-like_ substances have been identified in numerous organisms of this class as important constituents of the cell wall and of the capsule which is probably an outgrowth from the latter. Practically always associated with these substances are compounds containing nitrogen. One of these has been certainly identified as _chitin_ or a closely similar substance. Chitin is the nitrogenous substance which enters largely into the composition of the hard parts of insects, spiders and crustaceans. It is an interesting fact to find this substance characteristic of these animals in bacteria, as well as other fungi. Though it is extremely difficult to separate the cell wall of bacteria from the cell contents, in the light of our present knowledge it can be stated that the cell walls are composed of a carbohydrate body closely related to cellulose, though not true cellulose, probably in close combination with chitin. Of the organic constituents of the cell contents the most abundant are various proteins which ordinarily make up about one-half of the dry weight of the entire cell. The “Mycoproteid” of Nencki, 1879, and other earlier workers is deserving of little more than historical interest, since these substances were certainly very impure and probably consisted of mixtures of several “proteins” in the more recent sense.