DETAILS OF MITOSIS 



79 



attached to the chromosomes, and being, according to Hermann, the 

 principal agents by which the daughter-chromosomes are dragged 

 apart. The mantle-fibres thus form two hollow cones or half-spin- 

 dles, separated at their bases by the chromosomes and completely 

 surrounding the continuous fibres of the central spindle, which come 

 into view as the "interzonal fibres" during the anaphases (Fig. 28). 

 There is still considerable uncertainty regarding the origin and 

 relation of these two sets of fibres. It is now generally agreed with 

 Van Beneden that the mantle-fibres are essentially a part of the 

 asters, i.e. are simply those astral rays that come into connection 

 with the chromosomes — 

 wholly cytoplasmic in ori- 

 gin (Hermann, Driiner, 

 MacFarland), or in part 

 cytoplasmic, in part dif- 

 ferentiated from the linin- 

 network (Flemming, 

 Meves). Driiner ('95), 

 Braus ('95) (salamander), 

 and MacFarland {Plcuro- 

 pJiyllidia, '97) believe the 

 central spindle to arise 

 secondarily through the 

 union of two opposing 

 groups of astral rays in 

 the area between the 

 centrosomes. On the 

 other hand, Hermann 

 ('91), Flemming ('91), 

 Heidenhain ('94), Kos- 

 tanecki ('97), Van der 

 Stricht ('98), and others 

 believe the central spindle 

 to exist from the first in 

 the form of fibres stretching between the diverging centrosomes ; and 

 Heidenhain believes them to be developed from a special substance, 

 forming a " primary centrodesmus," which persists in the resting cell, 

 and in which the centrosomes are embedded.^ MacFarland's observa- 

 tions on gasteropod-eggs ('97) indicate that even nearly related forms 

 may differ in the origin of the central spindle, since in Pleurophyllidia 

 it is of secondary origin, as described above, while in Dianlula it is a 

 primary structure developed from what he describes as the "centro- 

 some," but which, as shown at page 314, is probably to be regarded as 



icy: p. 315. 



Fig. 30. — Mid-body in enibryonio cells olLiniax. [HOFF- 

 MANN.] 



Earlier stage above, showing thickenings along the line 

 of cleavage. Later stage, below, showing spindle-plate and 

 cytoplasmic plate. 



