HISTOLOGY AND CYTOLOGY 361 



Under the high powers of the microscope it will be seen that 

 protoplasm, in addition to showing the structure just noted, always 

 presents a granular appearance. This is due to its containing a 

 number of tiny particles that in fixed cells appear as solid granules, 

 and are of several different kinds. In the first place there are always 

 present an enormous number of minute granules, the microsomes, 

 which are distributed along the spongioplasm fibres, and appear to be 

 an essential constituent of the protoplasm. The cells of the lower 

 animals, particularly the Protozoa, often have granules of an easily 

 staining material chromatin, which is really, as we shall see later, a 

 constituent of the nucleus that has wandered out into the cytoplasm. 

 These chromidia, as they are termed, are not common in the cells of 

 the higher animals. Then, too, we have a number of slightly larger 

 granules not necessarily related to the spongioplasm that are the 

 result of the chemical activity of the cell, and may represent food 

 material, which is stored as a reserve or, as the result of the anabolic 

 changes, is on the way to being transformed into protoplasm ; or, 

 on the other hand, they are the katabolic products of the cell going 

 to form its secretions or waste matter. These are included in the 

 general term metaplasmic granules. In many cells, more particu- 

 larly the germ cells, are yet other groups of somewhat larger bodies 

 that may be of a spherical shape, when they are termed chrondo- 

 somes, or of an elongated or rod shape, termed mitochondria. They 

 can easily be seen after certain methods of fixing and staining, but 

 disappear after other fixing fluids. Different varieties of these 

 bodies have been made out, and the terminology employed to 

 describe them is not uniform, and sometimes contradictory ; but 

 for elementary purposes it is sufficient to note their presence, and 

 that they can perhaps be included under the terms given. Their 

 exact significance has not yet been ascertained, but it has been 

 shown that they play an important part in the activities of the cell, 

 and when the cell divides they appear to be more or less carefully 

 distributed to the daughter cells. In many cells we find certain 

 spaces filled with a clear fluid and termed vacuoles. In the case of 

 free-living cells these often represent the digestive or excretory 

 apparatus, as we have seen in the case of Amceba and Paramcecium. 

 Certain cells also contain cytoplasmic specialisations, to which we 

 can give the general name of plastids. They are most frequently 

 met with in plant cells, and are the centres at which some substance 

 is collected, e.g. the chloroplasts containing the chlorophyll, or the 

 amyloplasts where the starch is aggregated. 



There is also, in the cytoplasm, another compound structure 

 situated quite close to the nucleus, and, indeed, the evidence pro- 

 vided bv certain Protozoa seems to show that primitively it was 



