158 BROOKLYN BOTANIC GARDEN MEMOIRS 



all its dimensions. It results rather in a characteristic elongation in 

 one axis of the cell, the axes of elongation of the four cells tending to 

 be tangential to the general outline of the four-celled group. Klein 

 figures this elongation of the cells very clearly for Eudorina in the 

 four- and again in the eight-celled stage ('88, Taf. VI, Fig. 6i, 63). 

 Overton shows it more crudely ('89, Taf. II, Fig. 10) and I have been 

 able to photograph it for one of the four cells (Fig. 4, PI. II). It may 

 take place successivel}^ rather than simultaneously in the four cells and 

 apparently proceeds in either direction around the group. Biitschli's 

 figure ('83, Taf. XIV, i g.) shows rather crudely the resulting arrange- 

 ment of the cells just after the third division. The division seems to 

 be nearly simultaneous in all four cells and the wheel-formed group of 

 eight results. This characteristic growth and elongation of the cells 

 at this stage leads naturally to the oft-noted fact that the plane of the 

 third division cuts that of the second obliquely rather than at right 

 angles. A determining factor is, however, obviously the tendency to 

 bisect the elongated cell at right angles to its major axis as well as 

 the direct relation between the second and third cleavage planes. The 

 elongation of the cells during division reminds one at once of the 

 familiar elongation of the egg cell and other free globular cells at the 

 time when the bipolar karyokinetic spindle figure is at its climax of 

 development. We have no good figures of karyokinetic division in 

 Volvox but Overton's figure from a 200-celled colony ('89, Taf. Ill, 18) 

 shows telophase stages with the cells all elongated and the spindles 

 in every case in the long axes of the cells. 



It seems obvious that such a cell form in division implies a spindle 

 figure with polar asters and justifies the assumption that the same 

 internal forces are operating in the elongation of the Volvox cell as in 

 the dividing egg. Typical polar asters may be expected to be found 

 at such a stage as that shown in Overton's figure, like those shown by 

 Swingle ('97) for a corresponding stage of division in Sphacelaria. 

 We may conclude then that the adhesion of the four mother cells 

 makes it necessary that the movement of material preparatory to 

 the production of two equivalent rounded daughter cells should take 

 place upon their free surfaces and the result is the characteristic 

 bulging and elongation of the four cells during division. That this 

 change of form is associated with the production of the karyokinetic 

 figure with two polar asters seems clear from the figures of division 

 in other alga cells with centrosomes. In any one of the cells of the four- 

 celled group (Fig. 3, PI. II), for example, if the third spindle figure has 

 its axis 90° from that of the second division and in the same plane it is 

 obvious that one of the asters will not have space for its full expression 

 and if the adhesion of the quadrants is strong the yielding will be on 



