84 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the movements of the asters into the definitive position occupied by 
the spindle. 
(1)and (2). In regard to the first two questions, two general facts 
are worthy of notice. 
First, there is a tendency, other things being equaı, for the newly 
formed asters to separate at right angles to the axis of the preceding 
spindle, and in such a way that the asters are not to be distinguished as 
deep and superficial, but either as right and left, or dorsal and ventral. 
No explanation for this fact is apparent, and it is not in every case true. 
of the asters before the spindle is formed ; and (3) what determines 
Thus in the two-cell stage tbe nucleus and aster in the large cell migrate 
to the right before the aster divides, and the separation of the two 
newly formed asters is not in a plane at right angles to the axis of the 
preceding spindle. 
Secondly, the position of the asters before the formation of the spindle 
may apparently be modified by the simple mechanical conditions sur- 
rounding them. Thus, in the cell d*? (Fig. 14, Plate 2) the asters are 
modified in position almost immediately after they begin to separate, so 
that very soon we actually have in this cell the condition which may be 
considered least typical, — an inner and an outer aster (Fig. 7, Plate 1). 
In the thin cell d^? (Figs. 37 and 38, Plate 5, and 46, Plate 6) the form 
of the cell apparently operates to cause the two asters to separate in 
such a way that almost from the first the line joining them has the 
same direction as the axis of the preceding spindle. Such faets give 
the impression that before the formation of the spindle the position of 
the asters is undetermined, and indifferent for the general structure of 
the cell, except that the two asters always lie on opposite sides of the 
nucleus. 
(3) As the karyokinetic changes take place, the asters migrate into 
definite positions, apparently by a rotation of the whole complex com- 
posed of the nucleus and its two accompanying asters. This rotation 
is into a definite position, without regard to either the form of the con- 
taining cell, or the previous position of the asters; that is, the end to 
be gained is constant, while the means of gaining it vary. Thus, at the 
divisions to form the sixteen-cell stage, the line passing through the 
asters and nucleus rotates in d*! from a lateral to a dorso-ventral posi- 
tion, and into the greatest axis of its cell (Figs. 11, 15, and 16, Plate 2). 
In d*? it rotates from a position in which one aster is central, the other 
peripheral (Plate 1, Fig. 7), likewise into a dorso-ventral position (Fig. 
16). In a and dec all the axes rotate from a position of 
