442 ^A TA SE. [Vol. \' 1 1 1. 



the group of the filaments. I have no personal observation on 

 this subject, nor am I aware that such a view has ever been 

 suggested. But the figure I have copied from Strasburger 

 (Fig. 7) seems to favor such a view. The knob-like enlarge- 

 ment of the filament at its extremities (Fig. 7, X.), I consider 

 to be microsomes. Such a group of microsomes may be called 

 the polar microsomes, in distinction from the equatorial micro- 

 somes (Fig. 5, Eq. M.), which are found at the equatorial plane 

 of the spindle. 



If we suppose that these free ends of the spindle filaments 

 (Fig. 7) be brought together to a point, the thickened enlarge- 

 ments of each fibre will fuse with one another and form a 

 veritable centrosome, at each end of the spindle. 



The relation of the solitary centrosome at the end of the 

 ordinary spindle to the independent microsomes at the free 

 ends of the individual filaments of the truncated spindle, is 

 precisely the relation existing between the " Zwischenkorper " 

 and the equatorial microsome or "cell-plate." In the one the 

 material of the microsome is concentrated into one solid body, 

 and in the other it is distributed into several independent 

 pieces. 



In way of summary, we may say (i) that the cejitrosome is not 

 a unique organ of the cell, but is identical with the microsome 

 which exist everywhere in the cytoplasm. This view further 

 explains (2) the nature of the "cell-plate," which may also be 

 called the equatorial microsomes. (3) When such microsomes 

 at the equatorial plane of the spindle fuse into one solid body, 

 it is known as the ^^ ZzviscJienkorper!' (4) The knob-like 

 thickenings at the free-end of the spindle fibre as in Fig. 7, X, 

 are probably the microsomes. The barrel-shaped spindle 

 possesses, according to this view, several independent micro- 

 somes, instead of one centrosome, at each pole of the 

 caryokinetic figure. 



If these views be further substantiated by a future research, 

 the centrosome, far from being a unique structure, is one of 

 the most familiar constituents of the cell. The microsome, 

 centrosome, " Zivischenkorper,'" '^ cell-plate'' and \)ciQ. polar micro- 

 somes all belong to the same category of cytoplasmic material. 



