2/2 THE PROTOZOA 



takes place under normal conditions in Acanthocystis can be brought 

 about in Oxyrrhis marina, as Schaudinn has demonstrated, by placing 

 the flagellate in an abnormal medium. Ordinarily this form has an 

 intra-nuclear division-centre like that of Euglena, but if it is trans- 

 ferred to a more dilute salt solution, the substance of the division- 

 centre is forced out of the nucleus and into the cytoplasm, where it 

 swells to many times its usual size, and may even divide outside of 

 the nucleus, forming a large spindle in the cytoplasm (Schaudinn, '96). 



4. The so-called " Centrosomes " in Protozoa. 



In addition to the division-centres or spheres described above, 

 there are occasional centres in Protozoa resembling centrosomes 

 of the Metazoa. So little positive knowledge is at hand, however, 

 that the centrosome question in Protozoa must rest in abeyance. 

 A number of division-centres have been described as centrosomes, 

 but in almost all cases these are more like the spheres described above, 

 than the centrosomes of Metazoa. In a number of instances, however, 

 the primitive division-centres contain central, deeply staining granules 

 which form the foci of the spindle fibres, and these would seem to be 

 analogous to the centrosomes of higher forms. Such granules are 

 found in Spirochona^ Kentrochona, Actinosph&rium, and Noctiluca, and 

 probably they exist in other forms as well ; these, however, are the 

 only forms that have been critically examined. 



In Spirochona and Kentrochona, the granules in question appear only 

 at a late stage in division (Balbiani, '95 ; Doflein, '96), and they remain 

 in evidence until the next division period. In Actinosph&rium similar 

 granules appear during the maturation mitosis, but are not found at 

 other times (Hertwig). In Noctihtca they appear at each mitosis and 

 form the insertion points for the mantle-fibres (Fig. 141, F). 



The origin of these so-called centrosomes still remains obscure, 

 although in each case it has been maintained that they arise from the 

 chromatin. Plate ('86) and Butschli ('88) assumed that in the case 

 of Spirochona gemmipara they are formed from chromatin in solution, 

 while Balbiani ('95) maintained that they are actually granules of chro- 

 matin broken off from the polar ends of the chromosomes. Hertwig 

 ('98) described a similar origin in the case of Actinosphcerium, where the 

 granules are supposed to arise by budding of the chromatin (Fig. 140). 

 Brauer ('94), on the other hand, held that the centrosomes come from 

 the pole-plates. In Noctilnca, finally, Ishikawa ('94, '99) and Calkins 

 ('98) concluded that the centrosomes were nuclear in origin, while the 

 spheres are extra-nuclear at all times. The conclusion, however, could 

 not be supported by positive evidence and, at best, has only the value 

 of an assumption. 



