CELLS 499 



as we shall see later, many inconsistencies in our use of the term, 

 for it is sometimes applied to things with two nuclei or none. A 

 mass of protoplasm including two or more nuclei within one wall 

 is more strictly called a coenocyte, where it is the whole body 

 of an animal, or a syncytium where it is a part of the body. We 

 have seen examples of the former in Protozoa such as Paramecium 

 and some Amcchce, and shall meet the latter in tissues such as 

 muscle. 



CYTOPLASM 



It is often convenient to distinguish the material of which the 

 nucleus is composed as nucleoplasm and the rest of the cell as 

 cytoplasm. We shall now consider some of the properties of 

 these. 



Detailed investigation of the chemical nature of cytoplasm 

 can only be carried out by kiUing it, and we can be reasonably 

 confident that at death chemical changes occur, but there are 

 unlikely to be large transformations of one type of chemical 

 substance into another. Work on dead cytoplasm can therefore 

 give us useful information, and in addition a hmited number 

 of experiments can be done without causing death. Most of the 

 cytoplasm is water, and in this are present, in true or colloid 

 solution, several other substances ; the most important are the 

 proteins, which make up about sixty per cent, of the dry weight, 

 and others are carbohydrates, fatty substances or hpoids, and 

 inorganic salts and ions — the same things, in fact, as are needed 

 in the diet. Some other organic substances are present in small 

 quantities. In the cytoplasm are what are called inclusions of 

 two sorts ; some are protoplasmic, that is, are themselves living 

 and are chemically distinct specialisations of the cytoplasm, 

 and others, called deutoplasmic, consist of non-living material 

 formed by the cell. The chief of the protoplasmic inclusions, the 

 central apparatus, Golgi apparatus, and mitochondria, will be 

 referred to below. The deutoplasmic inclusions are granules, 

 crystals or globules of substances present in too great a quantity 

 to be held in solution ; the most obvious are the yolk granules 

 of Qgg cells, fat globules, and particles of pigment in many cells. 



The nineteenth-century workers all thought that cytoplasm 

 had a structure of visible fibres or surfaces, so that it was described 

 as a network or as a foam, but in fact these are the appearances 



