PROTOPLASMIC MOVEMENT. Sj 



carried from the left to the right side of each cell, the spindles 

 for their first subdivision appear, and the cleavage takes place 

 in a laeotropic direction (Fig. 13). This cleavage is a very 

 unequal one, the outer products being the small trochoblasts. 

 The inequality is preceded by a marked eccentricity of the 

 nuclear spindle (Fig. 13); this eccentricity is not indicated in 

 the position of the nuclei before the spindle appears, for they 

 lie near the inner angles of the cells (see the cell not dividing 

 in Fig. 13), but during the prophase the nuclei and spindles 

 move outward in the cells until the outer (lower) centrosome 

 comes almost into contact with the cell membrane, while the 

 inner (upper) one lies near the middle of the cell. In this 

 position the division takes place in a laeotropic direction (Fig. 

 13), and immediately the currents in the upper cell moiety 

 begin to rotate in a laeotropic direction, while those in the 

 lower moiety usually rotate in a dexiotropic direction. In this 

 case the eccentricity of the spindle is the immediate cause of 

 the unequal cleavage, and this eccentricity is the result of vor- 

 tical movements in the cell, which begin coincidently with the 

 breaking of the nuclear membrane. 



The first division of the second quartette is a nearly equal 

 one, and is slightly dexiotropic in direction. These cells were 

 formed by laeotropic cleavage, and consequently the rotation 

 within them is a laeotropic one. When this laeotropic move- 

 ment has carried the centrospheres from the extreme left to 

 the extreme right of the cell, the cleavage begins, the whole of 

 the sphere substance going into the right product of division 

 (Fig. 14). 



Especial interest attaches to the cell movements in reversed 

 cleavage, or cases in which two successive divisions are in the 

 same direction. Such an instance occurs in the first division 

 of the basal cells of the cross. (See Conklin, '97 and '98.) These 

 cells were formed by dexiotropic division of the apical cells, and 

 to preserve the law of alternation they should divide in a laeo- 

 tropic direction, but they all divide dexiotropically. Immedi- 

 ately after the apicals have given rise to these basal cells, the 

 contents of the former rotate in a dexiotropic direction until 

 the centrospheres are carried through an angle of about 90° ; 



