250 BOTANICAL GAZETTE [APRIL 
other Protozoa, a more or less clearly defined, fibrous, central spindle 
is found in addition to the mantle fibers. The absence of the central 
spindle in the simpler type of intranuclear division seen in Amoeba, 
Coccidium, and Empusa, and its meager development even in more 
complicated cases, clearly suggest that the central spindle-fibers, when 
present, play only a minor réle in nuclear division as maintained by 
HERMANN (’91), viz., that they are non-contractile supporting ele- 
ments, which form a basis on which the movements of the chromo- 
somes take place. The chromatic structures in Empusa are undoubt- 
edly moved poleward without the assistance of such connecting 
fibers, and they seem to be supported entirely by the surrounding 
nuclear sap. 
These facts may be interpreted as thus furnishing a strong argu- 
ment against the acceptance of the ‘“‘pushing theory” of DRUNER 
(’95), who supposes an active growth or elongation of the central 
‘spindle, thus pushing the spindle-poles farther and farther apart; 
and at least in part against the suggestion of MorriER (:03, :04) ,who 
thinks that the chromosomes may be conveyed to the poles both by a 
pushing and a pulling action of the spindle-fibers. 
No clear explanation of the mechanism which accomplishes these 
primitive divisions has yet been marked out. As pointed out above, 
there are in Empusa no specially differentiated mantle-fibers, since 
the radiating astral rays of the intranuclear figure themselves mark 
the paths of the chromatin-movement. Whether the movements 
which take place in these radiations are ienieeel to wee which ue 
in the aster of the more highly di 
I cannot say, but this seems quite probable. In Empusa the radiations 
extend in all directions from the centrosome and some are anchored 
firmly to to the persistent nuclear membrane at its nearest point, 
while others project into the nuclear cavity, apparently ending free in 
the karyolymph. Now, a contraction of the radiating fibers would 
undoubtedly accomplish just the phenomenon which we see takes 
place. The fibers seem to shorten and to thicken, and an appearance 
suggesting an accumulation of darker staining material immediately 
around the centrosome results. The distal indentation of the nuclear 
membrane which we see occasionally (figs. 64, 65) should also be 
regarded as strong evidence that a pull of some sort or a contraction. 
