THE NUCLEUS 79 



is probably based on incomplete or erroneous observation, or on 

 defective technique. In all Protozoa that have been examined in 

 recent times, at least one nucleus has been found to occur without 

 exception, though in some phases of the development the nucleus 

 may temporarily disappear and resolve itself into chromidia. 



There now remains for consideration the question of the centro- 

 some, the centre of the kinetic activity of the nucleus. Of all the 

 questions connected with the nuclear apparatus, those relating to 

 the centrosome are the most difficult to handle in a genera] manner, 

 largely on account of the minuteness of the bodies dealt with, and 

 the consequent difficulty of ascertaining their structure and com- 

 position, even their presence, in many cases. Hence, in the litera- 

 ture of the centrosome, there is found considerable confusion in 

 the terminology, different authors disagreeing entirely as to the 

 precise structures to which the name centrosome should be applied, 

 and opposed theories, which cannot be discussed adequately in a 

 short space, have been put forward as to the nature and origin of 

 the centrosome. 



As the focus of the kinetic activities of the nucleus, the centro- 

 some is most apparent and recognizable when the nucleus is in 

 process of reproduction by division, and much Jess so when the 

 nucleus is in the so-called "resting state." Hence the study of the 

 nucleus during the process of division is alone decisive as to the 

 presence of a centrosome in any given case ; and since in many 

 cases nuclear division appears to go on without centrosomes being 

 present, it may be taken as equally probable that, in all such cases 

 at least, no centrosome is present in the resting state of the nucleus. 

 In many cases, however, the presence of a centrosome in, or in 

 connection with, the resting nucleus can be ascertained clearly ; 

 it may then lie either outside or inside the nucleus. 



When the centrosome lies outside the nucleus, as it usually does 

 in the cells of Metazoa, it is found typically as a minute grain or 

 pair of grains (" diplosome ") close beside the nuclear membrane. 

 Its presence may be indicated by the radiate structure of the 

 surrounding protoplasm, giving the appearance of a system of rays 

 centred on the centrosome ; but such radiations are absent as a 

 rule during the resting state of the nucleus, and the appearance of 

 rays is often the first sign of impending activity and division of 

 the nucleus. In many cases the centrosome is found lying in a 

 mass of clear protoplasm termed archoplasm, a substance which 

 differs, apparently, from the rest of the cytoplasm only in being 

 free from granulations of all kinds. Archoplasm may, in short, be 

 regarded simply as pure cytoplasm, and it appears either perfectly 

 homogeneous, or traversed by striations which radiate from the 

 centrosome, through the archoplasm, and even beyond its limits; 



