300 Marquette, Manifestation of polarity iu plant cells which usw. 



Statements regarding the finding of the nucleoles near the spindle 

 poles durmg nuclear division also Tjelong here. The Observation 

 has been made by numerous investigators that the nucleoles. if 

 they persist long enough, frequently come to lie in the neighborhood 

 of the spindle poles. This has sometimes led to a confusion of 

 nucleoles with centrosomes. However, in not a Single case known 

 at present does the nucleole invariably take its position at the 

 spindle poles. more or less frequently it lies elsewhere in the 

 cell. This alone suffices to distinguish it from the centrosome. 

 The fact of the frequent oceurrence of the nucleoles at the 

 spindle poles reniains to be accounted for. however. Fischer 3 ) 

 made use of the Observation that the nuclear membrane in some 

 cases flrst disappears at the point nearest the spindle poles to 

 account for the polar position of the nucleoles. He conceived that 

 as the nuclear membrane disappears at the poles the liquid 

 contents of the nucleus escape at these points earrying the 

 nucleoles with them and as a result these come to lie near the 

 spindle poles. While this explanation is sufflcient to account for 

 the approximately polar position of the nucleoles it is entirely 

 inadequate to account for the exact and unvarying position of the 

 central bodies at the poles of the spindle to which Fischer also 

 applies this explanation. It is also still an open question whether 

 it is a general rule that the nuclear membrane breaks down flrst 

 in the polar regions. When the break does take place at these 

 points it seems probable that a tendency to bring the nucleoles 

 near the spindle poles will result, but it is entirely possible that 

 even where there is no such outstreaming of nuclear contents 

 towards the poles the nucleoles may nevertheless frequently lie near 

 the spindle poles. The same factors which bring about the 

 aggregation of chloroplasts, etc. in the neighborhood of the spindle 

 poles may also be responsible for the position of the nucleoles. 



Considerations like the preceding are. however. inadequate to 

 account for the position and behavior of the polar structures of 

 Fsoetes. In them we are not dealing with the aggregation of free 

 starch grains but with a body which at every stage oecupies a 

 specific position in the cell. It divides and its halves migrate 

 along definite lines and come to lie exactly at the poles of the 

 spindle. behaving in all these respects as does the centrosome. 

 The so-called „dynamic" theory of nuclear and cell division also 

 does not account for the behavior of the polar structures of Isoetes. 

 According to the dynamic theory there are especial structures 

 or. perhaps, only especial regions at the spindle poles from which 

 an attraction proeeeds, an attraction which at times at least 

 extends even to relatively inert bodies in the cell. In this way 

 the accuniulation of various cell-constituents at the spindle poles is 

 accounted for. Behind this view lies the coneeption of magnetic 

 attraction, for this. or a force acting according to similar laws as 



1 i Fischer. A. : Fbdrung. Färbung und Bau des Protoplasmas. Jena 1899 

 p. 248. 



