^^02 TH?: CELL AND MAMMALIAN HISTOLOGY 



CENTRAL APPARATUS 



Many different processes go on in cytoplasm, and accordingly 

 it has many specialisations of structure. The central apparatus, 

 or division centre, lies close to the nucleus and typically consists 

 of a small centriole surrounded by a centrosphere, the whole 

 being a centrosome, but there are variations on this plan. It 

 initiates cell division, dividing before the nucleus, and betv^een 

 its daughter halves develops the spindle, which in fixed cells 

 appears to be made up of fibres. The existence of these in the 

 living cell is doubtful, but that the spindle is a reality is shown by 

 the fact that it can be pushed to one side by a microdissection 

 needle and the fibres made to appear out of place. Similar fibres 

 which radiate outwards from the centrosome during nuclear 

 division are called astral rays. 



MITOCHONDRIA 



Mitochondria (sometimes called lipochondria) are filaments, 

 granules or spheres which are generally scattered through the 

 cytoplasm and are sometimes visible in the living cell. They have 

 been seen to move about, and look rather like bacteria. They 

 contain both lipoid and protein, and there is increasing evidence 

 that they are especially concerned with secretion. It is probable 

 that some enzymes, such as those concerned with oxidation 

 in rat liver and the amylase of Amoeba, are concentrated in 

 them. The Golgi apparatus, which also stains as if it consists of 

 both protein and fat, is a collection of rods and vacuoles, some- 

 times appearing to form a network, near the nucleus. It is said to 

 be concerned in the formation of yolk and in secretion. As Golgi 

 bodies have rarely been seen in living cells, and as hardly any two 

 workers agree on what they look like when fixed and stained, the 

 elementary student need not be concerned with them. 



Mitochondria are present in plant cells, but a Golgi apparatus 

 has never been satisfactorily demonstrated except in animals. 

 Plant cells typically contain a vacuolar system, where cytoplasm 

 is replaced by an aqueous crystalloid solution, the cell sap. It 

 begins as a network, and usually ends as a single large vacuole 

 occupying most of the volume of the cell. A vacuolar network 

 has been claimed to exist in some animal cells, and we have already 

 met contractile vacuoles in the Protozoa, but the relation of 

 these to each other and to the Golgi apparatus is obscure. Most 



