300 Marquette, Manifestations of polariiy in plant cells which usw. 
Statements regarding the finding of tlie nucleoles near tlie spindle 
poles during nuclear division also belong 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 lias sometimes led to a conf'usion 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 front the centrosome. 
The fact of the frequent occurrence of the nucleoles at the 
spindle poles remains to be accounted for, however. Fischer 1 ) 
made use of the Observation that the nuclear membrane in some 
cases first 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 nücleus escape at these points carrying the 
nucleoles with them and as a result these come to lie near the 
spindle poles. While this explanation is sufficient 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 first 
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 tliere is no such outstreaming of nuclear contents 
towards the poles the nucleoles may nevertlieless frequently lie near 
the spindle poles. The sarne factors wliich 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 
Isoetes. In them we are not dealing with the aggregation of free 
starch grains but with a bodv which at every stage occupies a 
specific position in the cell. It divides and its lialves 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 proceeds, an attraction which at times at least 
extends even to relatively inert bodies in the cell. In this way 
the accumulation of various cell-constituents at the spindle poles is 
accounted for. Behind this view lies the conception of magnetic 
attraction, for this, or a force acting according to similar laws as 
x ) Fischer, A.: Fixirung, Färbung und Bau des Protoplasmas. Jena 1899 
p. 248. 
