362 AN INTRODUCTION TO ZOOLOGY 



actually a part of the nucleus itself, within which it is to be found 

 in some forms. In the cells of higher animals, however, it lies 

 outside the nucleus, and so can be considered as a cytoplasmic 

 inclusion. It takes the form of a tiny spherical granule, termed the 

 centriole or centrosome, that stains very intensely with certain dyes 

 and so stands out quite clearly. Often in higher forms it is not 

 single, but a pair of granules side by side, and it is then termed the 

 diplosome. Strangely enough this granule, so constant in animal 

 cells and, as we shall see later, playing such a notable part in the 

 process of cell division, has not been shown to be present in plant 

 cells, and so its presence constitutes one of the most striking and, up 

 to now, inexplicable differences between the cells of animals and 

 plants. It is surrounded by a small sphere of very clear, apparently 

 structureless protoplasm, spoken of as archoplasm, while the sphere 

 itself is termed the centrosphere or attraction sphere. This, then, 

 completes the list of the main structures to be found in the cytoplasm. 



The nucleus in the higher animals is in the form of a sphere or a 

 more or less elongated ovoid, but in the Protozoa it may be of a 

 very irregular shape, and in some cases divided up into two parts 

 of different functions. The nucleus, as a whole, plays a very im- 

 portant part in the activities of the cell, and appears to be the 

 controlling centre of the cell. Not only does it seem responsible 

 for the chemical changes in the cytoplasm, particularly those leading 

 to assimilation, but it also takes the initiative in the processes leading 

 up to cell division. 



The most important constituent of the nucleus is a material 

 termed chromatin, which is generally distributed throughout the 

 nucleus in the form of granules and threads. In lower animals 

 some of it may be extranuclear in position. It receives its name 

 from the fact that it very readily takes up certain basic dyes, such, 

 for example, as hsematoxylin. Its full significance will be more 

 readily appreciated when we come to consider the part it plays in 

 division and in the activities of the germ cells. Chemically, 

 chromatin is noticeable because it contains nucleic acid, an organic 

 acid rich in phosphorus. Another important part of the nucleus 

 is the linin or achromatin, so called because of the difficulty with 

 which it can be induced to take up stains. This is in the form of a 

 fine interlacing network of fine fibres with which the chromatin is 

 closely associated. The nucleus is separated off from the cytoplasm 

 by a very thin, homogeneous nuclear membrane, which also seems to 

 be made of the same achromatinic substance and is difficult to stain. 



Two other substances are present in the nucleus which also 

 stain readily and very similarly to chromatin. The first of these is 

 volutin, which with special treatment stains slightly differently from 



