258 THE PROTOZOA 



E. KINETIC STRUCTURES 



The confusion which has arisen concerning the relation of the 

 centrosome to the surrounding structures in Metazoa, and the uncer- 

 tainty which arises from an ambiguous terminology, are magnified 

 many times when we come to consider analogous structures in Proto- 

 zoa. It is well to say at the outset that in the Protozoa there 

 are but few structures that can be compared with the complex astral 

 systems such as those in the leucocyte, or at the spindle-poles of 

 dividing animal cells. If, however, we regard the centrosome (in- 

 cluding the centriole) and attraction sphere as a unit, and as represent- 

 ing a structure concerned in cell-division, we have a basis for 

 comparison with structures in Protozoa which play a corresponding 

 role. In the present chapter, I shall, for the sake of brevity, speak 

 of these structures in Protozoa as the "division-centres." They 

 are not always of definite form and size, although consisting prob- 

 ably of an analogous substance or substances. In some cases the 

 material or substance which corresponds to that in a more defi- 

 nitely formed division-centre, is apparently spread throughout the 

 nucleus, and in other cases even into the cytoplasm. It will be 

 convenient, therefore, to speak of the " division-centre " not only as 

 a body, but also as a substance which is intimately connected with 

 mitosis. 



In some cases the division-centre is intra-nuclear, in others extra- 

 nuclear, and in still others it may be sometimes one, sometimes the 

 other. In some of the more primitive Protozoa, notably in some of 

 the marine Rhizopoda, and in a few Sporozoa, there appears to be no 

 such element present during the resting periods of the cell. Dur- 

 ing division, however, there appears to be a substance which is 

 derived from the chromatin and which functions as a division-centre. 

 This is particularly interesting in the case of Monocystis ascidice, 

 recently described by Siedlecki ('99). Here, during the formation of 

 the conjugating gametes or sexual reproductive bodies, there appears 

 in the nucleus a distinct, deeply staining granule which divides while 

 against the inner nuclear membrane, to form two division-centre?. 

 These become the poles of the division-figure, and during division, 

 there is a connecting strand of deeply staining material. There 

 are other cases, also, especially among the Sporozoa, in which 

 a division-centre appears during nuclear division, although it cannot 

 be made out in the cells when at rest. Thus, I have seen a faintly 

 striated spindle in the dividing nucleus of a polycystid Gregarine 

 of the leech Clepsine (Fig. 138), while the resting nucleus shows no 

 trace of a substance similar to that forming the spindle, unless, indeed, 



