NUCLEAR DIVISION IN PROTOZOA. 211 



mosomes strung upon them, while central spindle fibers, if 

 present, must be intermingled with the others, and both sets 

 must have the same origin (Wilson, '95), Toxopneustes^ In 

 many cases of this nature, especially in the huge mitotic fig- 

 ures of the first cleavage nucleus of many eggs, the spheres 

 are relatively enormous, while the centrosomes are small and 

 often difficult to find (ToxopneustfS and Arbacia). Finally the 

 third type of mitosis differs from the first in the absence of 

 central spindles and centrosomes, and from the second type 

 by the absence of centrosomes (higher plants). 



While opinion differs as to the presence or absence of cen- 

 trosomes in plants higher than the fungi, some observers deny- 

 ing, others affirming, its presence in the same species, the 

 balance of opinion at present appears to be towards the nega- 

 tive side, and evidence is certainly accumulating to support 

 this view. Nevertheless, the difficulty of proving a negative is 

 nowhere more apparent than here, and one positive affirmation 

 throws down the entire conception. On the other hand, posi- 

 tive assertions are excessively dangerous, many plastids, gran- 

 ules, or other products of the cell being easily mistaken for 

 centrosomes, and only the most far-reaching experiments, with 

 the best technical skill, can safely determine their presence. 

 According to numerous observations by Strasburger, Mottier, 

 Osterhout, etc., the spindle in plant mitoses arises by the grad- 

 ual convergence of rays which make their appearance tangential 

 to the nuclear membrane. Arising, as it were, from the sub- 

 stance of the cytoplasm, and converging to a bipolar mitotic 

 figure, the spindle fibers are supposed, by Strasburger and his 

 followers, to be composed of a definite and distinct substance 

 to which he gave the name " Kinoplasm." The nuclear mem- 

 brane here, as in the other types, always disappears before the 

 nuclear plate is formed, and nuclear division proceeds in the 

 usual way. 



Leaving out of consideration for the present the discussion 

 of the many intermediate forms of resting nuclei which, among 

 the Protozoa, approach the type of nucleus of higher forms, we 

 find in this primitive group so many variations of mitosis, 



1 Journ. of Morph.) vol. xi. 



