28 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The asters in all of the eight cells after the third cleavage separate 

 tan"entially, at right angles to the direction of the preceding spindles. 

 Having tulieu np positions on opposite sides of the nucleus, in every 

 case the complex of nucleus and asters rotates in sucii 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 a^---c'''^ and in d*-^ is markedly unequal. 



In two cells of equal ages but unequal size (c?*-^ and d^-^) the lai-ger 

 divides first. 



Tlie 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 a*-^ in the same figure.) As the spindle nar- 

 rows and lengthens and the chromosomes begin to separate, the cell 

 continues to elongate (Fig. 2G, a*\ compared with Fig. 20, c*-^, 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 

 o<-2, in Plate 4, Fig. 28.) 



As Heidenhain ('94', 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 tlien 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 sigTiificance. 



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. 



