84 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



of the asters before the spindle is formed; and (3) what determines 

 the moveraeuts of the asters into the de6nitive position occupied by 

 the spindle. 



(1) and (2). In regard to tlie first two questions, two general facts 

 are worthy of notice. 



First, there is a tendency, other things being equal, 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 Yeutral. 

 No explanation for this fact is apparent, and it is not in every case true. 

 Thus in the two-cell stage the 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 riglit 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}-^ (F'g- 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 facts give 

 the impression that before the formation of the spindle the position of 

 the asters is undetermined, and indiff"erent 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 rf**^ 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). Ina*'^-c*-^ and a'^-^-c^''^ all the axes rotate from a position of 



