608 
T. H. MORGAN 
position for the first cleavage? Do the nuclei move back independ- 
ently of the spindle? Does the spindle first form near the nuclei 
and then move into position? Sections give a clear answer to 
these questions. Eggs were killed and preserved at close inter- 
vals after the nuclei had been driven into the excentric position. 
Sections of some of these eggs are shown in plate 5, figs. 14- 
22. It is clear from their figures that the nuclei do not move 
back to the center of the egg but resolve themselves in place; 
even before they break down, the chromatin begins to move 
towards that side of the cell nearest the aster. The fibres of the 
aster are attached at this point. There is no evidence that they 
penetrate as yet the nuclear field, but there is abundant evidence 
that the chromatin in some way draws to the asters and 
later enters the spindle. In the early stages of my work I had 
obtained some eggs in which the chromosomes seem to be drawn 
out from the centers towards the oil cap as in the egg in plate 
4, fig. 11, and this led me to think that they had been moved. It 
is now clear that the resting nuclei had been moved and the chro- 
mosomes were drawing over towards the asters. 
In another and better way I studied the same phase of develop- 
ment. When two pronuclei were present the eggs were put on a 
water centrifuge and kept there until the segmentation had begun 
in some cases. Were it possible for the pronuclei to move back 
when the eggs were taken from the centrifuge, it would not be 
possible for them to do so, I argued, if kept all the time on the 
machine. In fact the nuclei remained in their forced position. 
Nevertheless a spindle formed and cleavage took place, the cleav- 
age plane passing through the animal pole of the egg. Eggs treated 
in this way have been extensively studied, and in fact the figures 
of plate 5, 13-22 are from these eggs. The movement of the 
chromatin towards the aster is clearly seen in figs. 16, 17, 18. 
The movement of the spindle into position after it has accumulated 
its chromatin is also evident in many of the sections. The spindle 
may move towards any point of the egg without reference to the 
presence there of oil or yolk. It appears to assume its normal posi- 
tion in regard to the polarity of the egg. The spindle moves de- 
spite the fact that it is on the centrifuge during this time. It is 
