SO EDWIN G. CONKLIN 
a spindle moves out of a protoplasmic field into the yolk in order 
to reach its normal position in the cell. When eggs like the one 
shown in fig. 11 are removed from the centrifuge, the egg and sperm 
nuclei, together with the cytoplasm surrounding them move up 
through the yolk until they ultimately lie in their normal position 
on the animal side of the egg, beneath the polar bodies. How- 
ever far the germ nuclei or the first cleavage spindle may be re- 
moved from the chief axis of the egg, they invariably come back 
to their normal positions, with the equator of the spindle in the 
egg axis, and the long axis of the spindle at right angles to the egg 
axis, unless the spindle is held so long in its abnormal position 
that it is caught in that position by the divisional processes. The 
same is true also of the nuclei and spindles of the 2-cell stage; 
when moved out of the median plane of the cell they come back 
to that median plane, unless the cells are injured or the spindles 
are held in their abnormal position until the metaphase or a little 
later. Evidently the cause of equal cell division, such as the first 
and second cleavages of Crepidula, is not so simple as those have 
assumed who have attributed it to pressure, the line of least 
resistance, or the long axis of the protoplasmic mass. 
Not only the eccentricity or lack of eccentricity, but also the 
axis of the spindle is of great importance in determining the char- 
acter of the cleavage. While the former is associated with the 
equality or inequality of division, the latter conditions its differ- 
ential or non-differential character. The polar differentiation 
of the egg is the first visible morphogenetic differentiation, and it 
is not without significance that in the first and second cleavages 
of the egg the spindles are at right angles to the egg axis, while 
in the third, fourth and fifth cleavages they are more nearly par- 
allel with that axis. 
I have hitherto spoken of the position of the spindle as if it 
were the one cause of equal or unequal, differential or non-differ- 
ential cleavage; but for many reasons it is evident that the posi- 
tion of the spindle is itself the result of the structure or organiza- 
tion of the protoplasm, and that in this organization polarity and 
symmetry play an important part. Many years ago (’93) I 
showed that even before the spindle is formed, the shape of the 
