38 THE STRUCTURE OF CELLS 



in them an analysis of the processes may render it possible to dis- 

 tinguish between the essential and what is merely accessory, and 

 may indicate the mode and directions in which the structures 

 characterising the higher plants and animals have been elaborated. 

 The essence of the type may perhaps be most clearly gathered from 

 a consideration of the deviations from it. Nevertheless, one is 

 confronted at the outset with difficulties. For although the nuclei of 

 many protozoa are apparently extremely simple, yet in the details of 

 their division they may exhibit considerable complexity, and this not 

 by any means always in the direction followed by the nuclei of higher 

 organisms. And conversely, nuclei are not seldom met with in 

 these low organisms which surpass those of the metazoa and meta- 

 phyta in differentiation, whilst in mitosis they are commonly simpler. 



The features which seem to be common to all nuclei are (1) the 

 existence of chromatin in some form or other ; (2) a matrix in which 

 the chromatin is imbedded, but which in the simplest cases may be 

 indistinguishable from the ordinary cytoplasm. Furthermore, the 

 fission of the chromatin is a common, perhaps invariable, antecedent 

 to nuclear division, but it is often difficult to ascertain, and may 

 possibly be really absent in some cases ; for example, in many amitotic 

 divisions. 



The subsidiary structures, amongst which the centrosome stands 

 pre-eminent, can only be rightly appraised when their origin has 

 been traced in the lowest forms, in which various bodies which 

 appear to possess functions analogous with those credited to 

 centrosomes have often been distinguished. 



As regards the occurrence of nuclei in the Protozoa and the 

 simplest plants, the investigations of recent years have tended to 

 reduce the number of those from which nuclei were formerly 

 believed to be absent, and at the same time it has become evident 

 that the structure in question may be present in very different 

 degrees of completeness. Thus in Chromatium, and probably in 

 bacteria generally, it is not possible to speak of the existence of a 

 definite nuclear body, but granules which on good grounds have 

 been identified with chromatin are to be distinguished in the 

 protoplasmic framework of the cell. In the cyanophyceae also 

 similar granules are visible, but are restricted to a definite 

 specialised part of the cell-protoplasm, although the latter cannot 

 be spoken of as a nucleus. In many of the forms which possess 

 scattered chromatin granules there is visible in the cell a body 

 which is obviously connected with the mechanism of chromatin 

 distribution, for on cell-division the granules of the latter congre- 

 gate around the central body, which sooner or later divides, each 

 half carrying with it half the chromatin, in the form of attendant 

 satellites, to each daughter cell. In some organisms, e.g. 

 Tetramitus, the granules are distributed through the cell- 



