JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 81 
demanded by the principle of least surfaces. Flat plates, such as we sce 
in d”? and de, and in all the cells of the third layer, retain their form 
in virtue of some force working strongly against surface tension. 
After the fourth cleavage, the asters in all the cells at first separate 
at right angles to the axes of the preceding spindles, as happened after 
the third cleavage. The later changes are essentially the same in all 
the quadrants, so far as the asters are concerned, so that quadrant D 
may be described as a type. 
Figure 31 (Plate 4) shows the conditions in the four cells of quadrant 
JD, after the asters have divided. The two asters of each cell lie upon 
opposite sides of their nuclei in such a position that, if no change 
occurs, the ensuing division will be meridional. 
In the cells of the first three layers the asters retain their original 
positions. But in ds a rotation takes place, such as occurred in all the 
cells in preparation for the preceding (fourth) cleavage, so that the axis 
of the spindle in d9? is at right angles to the axes of the spindles in the 
other three cells of the quadrant. This condition is shown in Figure 
33, and the completion of the division is shown in Plate 5, Fig. 37. 
The same processes take place in the other quadrants, so that all tho 
cells of the first three layers have spindles extending laterally, while in 
the fourth or dorsal layer the spindles are directed dorso-ventrally 
(Fig. 40). 
We must now eonsider the cleavage in the several cells more in 
detail. 
As in previous cleavages, division takes place first in the cells of 
quadrant D. The nucleus of the large ventral cell, d??, is earliest to 
enter upon the karyokinetie process, followed immediately by d°*, and 
a little later by dë? and d. Figure 33 (Plate 4) gives a view of the 
posterior surface of the ege at this stage, showing the spindles in all the 
cells. As this figure shows, the spindles in the three cells d, d5*, 
and d? do not lie in the middle of the cells, but nearer the right ends. 
(See definition of right and left, page 17.) In the dorsal cell, &., the 
spindle is at right angles to those in the other cells. The plane of 
cleavage indicated in the three ventral cells is meridional ; in the dorsal 
cell it is equatorial, like that of the two preceding cleavages. The division 
of ench of the four cells must be considered separately. 
To understand the cleavage of the large ventral cell, 457, it is neces- 
sary to observe accurately its position and relation to the other cells. 
A longitudinal section of about the same stage as that shown in Figure 
33 is given in Figure 34. Comparing this with the earlier correspond- 
