28 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
The asters in all of the eight cells after the third cleavage separate 
tangentially, at right angles to the direction of the preceding spindles. 
Having taken up positions on opposite sides of the nucleus, in every 
case the complex of nucleus and asters rotates in such a way as to bring 
the axis of the forming spindle into the same direction as that occupied 
by the spindle for the preceding cleavage. In six of the cells, this ro- 
tation is from a previous position in the longer axis of the cell to a later 
position in the shorter axis. In these six cells spindles are completed in 
the shortest axes of the cells, and division ensues in such a way that 
the newly formed septa are surfaces of greatest area, and the cells sepa- 
rate in the direction of greatest pressure. 
The cleavage in d - c*? and in de is markedly unequal. 
In two cells of equal ages but unequal size (4% and de) the larger 
divides first. 
The exact changes in form during the divisions of the cells is a point 
worthy of careful attention. As the transformation of the nucleus 
giving rise to the spindle takes place, the cell elongates slightly in the 
direction of the spindle. (Compare c, Figure 24, with the earlier 
stage of the similar cell d in the same figure.) As the spindle nar- 
rows and lengthens and the chromosomes bogin to separate, tho cell 
continues to elongate (Fig. 26, , compared with Fig. 20, %, and Fig. 
22, a^). As the two new nuclei are formed and move apart, and the 
cytoplasm becomes constricted, there is a still further extension of the 
cells in the direction of the spindle. (Compare c and c*? with a*! and 
a*?, in Plate 4, Fig. 28.) 
As Heidenhain (94e, p. 154) has recently urged, this elongation of 
the cell in the direction of the spindle is a point of great importance 
for a proper understanding of the conditions affecting the direction of 
cell division. In many later divisions in Asplanchna the spindle is first 
formed, as will be shown, in the short axis of the cell, and then this 
axis by stretching becomes the longer. It is possible that to this phe- 
nomenon is due the apparent general agreement of normal cleavage with 
the law of Hertwig, and that careful observation will in many cases, 
as in Asplanchna, show the so called law to be of little significance. 
A full discussion of the bearing of the facts above described is 
reserved until later cleavages have been examined. 
The foregoing description is based on a study of forty-two specimens 
from different individuals, showing the various phases of the fourth 
cleavage ; that is, each containing more than seven and less than six- 
teen cells. 
