618 
T. H. MORGAN 
to be very greatly altered to include the cytoplasm of these regions 
in addition to the introduced substances. 
The evidence seems to me well-nigh conclusive that the size of 
the cells is not determined by regional differences but by the 
karyokinetic spindle, and its surrounding materials. It has been 
shown that when a spindle moves into the yolk or oil it displaces 
these materials to a certain extent. In such cases it may appear 
that the materials introduced from the interior of the egg with 
the spindle are responsible for the proportionate sizes of the cells. 
But the amount introduced is small compared with the cell as a 
whole which may still contain a large amount of oil or yolk, so 
much so in fact that in the living egg it appears to be entirel}^ 
filled at times with one or the other of these materials. The 
question that arises here may, however, be met by those cases in 
which the egg is centrifuged after the segmentation spindle has 
been formed. In consequence the yolk or oil may be closely 
aggregated around the outer pole of the spindle, yet the division 
that follows is into two unequal cells of proportionate size. 
It shouldbe recalled that the karyokinetic figure occupies practically 
the whole cell. By driving oil or yolk to one endof the spindle the fibre 
system is left largely intact. It is this system that appears to he the 
determining factor in the division, and not the materials that fill 
the rest of the cell. On this view, and this alone, can we understand 
how proportionate cleavage takes place under the conditions intro- 
duced by the centrifuge. This conclusion brings into the foreground, 
I believe, the important role that the karyokinetic figure plays in 
division, and conversely directs attention away from the chromatin, 
the nuclei, and the remainder of the cytoplasm that does not take part 
in the spindle formation. The conclusion shows that theories of cell- 
division that assume the nucleus to be the controlling factor, or its 
chromatin, and the theories that rest on cytoplasmic regional differ- 
ences apart from the spindle may have ignored the essential factor 
in cell division — the karyokinetic figure. 
