80 THE PROTOZOA 



the striations themselves being the optical expression of a radiate 

 arrangement of the protoplasmic alveoli (meshes of the " retic- 

 ulum "), indicating lines of force or tension centred in the centro- 

 some. In some cases it is probable that archoplasm showing 

 radiate striations may be present without any centres o me. In 

 Actinosphcerium Hertvvig showed that rays were formed in the 

 archoplasm before a centrosorne had been formed, and heralded its 

 appearance. 



When the centrosome lies within the nucleus, it is found most 

 frequently, in Protozoa, within a plastin-body or karyosome, a 

 position which it may retain permanently during both the resting 

 and dividing conditions of the nucleus. The simple nuclei of 

 the protokaryon-type probabty contain in most cases a centro- 

 somic grain lodged in the karyosome. In a few cases, however, 

 an intranuclear centrosome occurs without a karvosome, or outside 



*/ * 



the karyosome if one is present. On the other hand, there are 

 many examples of the occurrence of extranuclear centrosomes in 

 Protozoa ; but these are for the most part cases in wliich the centro- 

 some is in relation, not only to the kinetic functions of the nucleus, 

 but also to those of other cell-organs, as will be described presently. 

 Nuclei containing centrosomes have been termed '' centronuclei ' 

 by Boveri. 



The centrosome is seen, as a general rule, under the form of a 

 minute grain, or centriole. This is the form in which it occurs 

 invariably when it has an intranuclear position, lodged within the 

 karyosome. But when it occurs outside the nucleus, it exhibits 

 structural peculiarities which may vary at different periods, and 

 it often presents cyclical changes corresponding to different phases 

 of the activity of the nucleus. Thus, in Actinosphcerium, Hertwig 

 (64) describes the centrosome at its first appearance as a relatively 

 large body of spongy structure, formed at one pole of the nucleus from 

 extruded portions of the achromatinic reticulum (Fig. 37, A E). 

 At this stage, in which the centrosome is termed a centrosphere , it 

 lies in a patch of archoplasm, and is the centre of a well-marked 

 system of radiations. The centrosphere then gives rise, by con- 

 densation of its substance, to two centrioles, or to one which divides, 

 and at the same time the archoplasmic radiations become fainter 

 and disappear (Fig. 37, F, G). The centrioles then take part in 

 the division of the nucleus, and when this process is complete they 

 again become spongy centrospheres, which go through the same 

 series of successive changes that have already been described. Ana- 

 logous cyclical changes of the centrosome have also been described 

 in other cases, and have led to a conflict of opinion as to whether 

 the term " centrosome " should be applied to the whole centrosomic 

 complex, as it may be termed, or to the centrioles, of which many 



