.606 
T. H. MORGAN 
seen other spindles in the metaphase that were not displaced, and 
also other stages in this division that showed no such displacement. 
Eggs centrifuged while the second polar body is being given off 
offer two points of interest. The yolk is driven sometimes into the 
polar region, pi. 2, fig. O. The outer end of the spindle is obscured 
by the yolk; at best it is not well developed when the spindle is 
near the surface. It will be noted that the yolk is driven into the 
region all around the spindle but not within the spindle itself. In 
other words the materials that surround the spindle are permeable 
to the yolk spheres ; at least when as in this case the spindle is in a 
late phase of division. It is probable that the egg weighted by the 
polar body tends often to orient as it falls: at least I have rarely 
found the oil cap in the polar field but this may be due to the polar 
spindle becoming displaced by the oil. 
The egg quickly reaches the stage where two pronuclei are pres- 
ent. At their fullest development these nuclei are very large com- 
pared with the rest of the egg, plate 4, fig. 10, and lie near its mid- 
dle. During these stages the two pronuclei are easih^ moved by 
means of the centrifuge. As shown in plate 2, in fig. R, and 
plate 4, fig. 11, 12; plate 5, 13, 17, 24; they are carried beneath the 
oil cap, touching it on their outer ends, but not penetrating it. 
The specific gravity of the pronuclei is obvious from their posi- 
tion. They are heavier than the oil, lighter than the yolk, and 
lighter even than the cytoplasm . 
A large number of experiments were made at this stage of de- 
velopment in order to study the problem of the formation of the 
segmentation spindle when the nuclei are carried into foreign parts 
of the egg. While nuclei in the resting stage may be moved, my 
experience has been that in Cumingia asters and spindles can 
rarely be moved unless the protoplasm about them is shifted 
bodily. It follows that the centrosome (and its aster) brought in 
b}^ the sperm will be left where it lies when the nucleus is removed, 
unless the two are attached by the rays of the aster. In the latter 
case it is possible that the nucleus would be swung around and 
carried to the oil cap drawing the aster after it through the egg. 
Something like this appears to happen, although the point 
is difficult to settle positively, owing to the different possible posi- 
