THE REPRODUCTION OF THE PROTOZOA 103 



elements, assumes a characteristic bipolar form, like a spindle. 

 At each pole of the spindle a centrosome or centriole is to be found, 

 as a general rule. The two centrosomes have arisen by division of 

 the originally single centrosome, and may remain for some time 

 connected by a fibril or by a system of fibrils, forming what is often 

 termed a ' ' central spindle, ' ' but is better named a centrodesmose. The 

 axis of the achromatinic spindle is formed by the centrodesmose, if it 

 persists, and the remainder of the spindle is constituted by the so- 

 called ' * mantle-fibres ' ' running from pole to pole. The mantle-fibres 

 are derived from the achromatinic reticulum of the nucleus and the 

 archoplasm ; they are probably in most cases the optical expression 

 of an arrangement of the protoplasmic alveoli in longitudinal rows, 

 under the influence of tensions or forces centred at the poles of the 

 spindle. Such an arrangement of the alveoli produces the optical 

 appearance of fibrils connected by cross-junctions, the apparent 

 fibril being formed by thickened walls of alveoli in line with one 

 another, while the cross-junctions are the transverse walls between 

 consecutive alveoli. On this view the apparent fibrils of the achro- 

 matinic spindle are in reality merely the indication of lines of force 

 in the protoplasmic framework ; but some authorities consider that 

 in certain cases at least true fibrils are formed, which may be 

 isolated from each other and without cross-connections (Hertwig, 

 64). The spindle-fibres, whether real or apparent, are centred at 

 the poles of the spindle on the centrosomes, from which other 

 striations may radiate out in all directions through the archo- 

 plasmic masses (" attraction-spheres "), and extend into the sur- 

 rounding cytoplasm. 



While the achromatinic spindle-figure is in process of formation, 

 the chromatin of the nucleus has gone through a series of changes 

 which may differ in different cases, but which result in the forma- 

 tion of a number of masses of chromatin termed chromosomes. The 

 number, size, and shape, of the chromosomes vary greatly in dif- 

 ferent species, but in Metazoa these characters are generally con- 

 stant for the corresponding phases of the same species. Each 

 chromosome, when formed, consists of a great number of minute 

 grains of chromatin, chromidiosomes, cemented together in a matrix 

 or ground-substance of plastin. The chromosomes arrange them- 

 selves at the equator of the achromatinic spindle in the form of a 

 plate, hence termed the equatorial plate. The nucleolus disappears, 

 being absorbed or cast out, and does not contribute to the karyo- 

 kinetic figure, but a part at least of its substance probably furnishes 

 the plastin ground-substance of the chromosomes. 



At this phase, when the achromatinic spindle is fully formed, with 

 the plate of chromosomes at its equator, the actual partition of the 

 chromatin between the two future daughter-nuclei usually begins, 



