248 THE PROTOZOA 



ing from the centrosome and pushing in the nuclear membrane, which 

 finally disappears, leaving the chromosomes in the cytoplasm. The 

 mantle-fibres then become attached to the chromosomes, and the 

 latter, finally, surround the central spindle like a ring. The origin of 

 these fibres is variously interpreted. The mantle-fibres according to 

 some observers arise from cytoplasmic material, according to others, 

 from linin substance in the nucleus. The origin of the central 

 spindle-fibres is also in dispute. Some observers believe that two 

 opposing groups of fibres starting from the centrosomes meet to form 

 the spindle (Driiner, '95 ; MacFarland, '97; etc.); others believe that 

 the diverging centrosomes are connected from the first by the cen- 

 tral spindle-fibres (Hermann, '91; Flemming, '91; Heidenhain, '94; 

 Kostanecki, '97; etc.), and Boveri ('88) and Heidenhain maintain 

 that the fibres are formed from the substance surrounding the cen- 

 trosome (archoplasm of Boveri; substance of the "centrodesmus" of 

 Heidenhain). 



Several transitional stages between the first and the second types 

 have been described. In some of these (many egg-cells), the central 

 spindle appears first as very delicate fibres between the dividing 

 centrosomes, but these break later, and no connection remains. The 

 complete spindle in such a case consists of fibres which apparently 

 pass from pole to pole, with chromosomes strung upon them, while 

 central spindle-fibres, if present, must be newly formed and inter- 

 mingled with the other fibres, and both sets must have the same origin. 1 



Finally, the third type of mitosis differs from the first in the ab- 

 sence of central spindle and centrosomes, and from the second by the 

 apparent absence of centrosomes. While great difference of opinion 

 exists as to the presence or absence of centrosomes in plants higher 

 than the Fungi, some observers denying, others affirming, its presence 

 in the same species, the balance of opinion at present appears to be 

 toward the negative side, and evidence is certainly accumulating in 

 support of this view. According to Strasburger, Mottier, Osterhout, 

 etc., the spindle in plant mitoses arises by the gradual convergence 

 of rays which make their appearance tangential to the nuclear mem- 

 brane. Arising, as it were, from the substance of the cytoplasm, and 

 converging to form a bi-polar mitotic figure, the spindle-fibres are sup- 

 posed by Strasburger and his followers to be formed from a definite 

 and distinct substance to which he gave the name Kinoplasma. The 

 nuclear membrane, as in the other types, always disappears before the 

 nuclear plate is formed, and nuclear division proceeds in the usual way. 



Turning now to the Protozoa, the heterogeneity in form and struc- 

 ture of the nuclei is particularly suggestive. All of the several parts 



!Cf. Wilson ('96), Toxopneustes. 



