JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 4T 
three quadrants given on page 41, shows that the directions of the cell 
walls are tho same in both, the inequality in the size of the cell in 
the quadrant D being the only difference. 
In the seventh cleavage, the spindle appears first in the cell d7^, as 
shown in Figure 58, and the cell is divided meridionally into two equal 
cells, d? and dee. The finished division is shown in Plate 8, Figs. 
66, 67, and 68. The plane separating these two cells is the median 
dorso-ventral plane of the embryo, as will be shown later. 
Shortly after this division is completed, spindles appear in CRIS AE 
dun, %s, and diu, as shown in Figure 66. In da’ the spindle is 
dorso-ventral, hence lying in the shorter axis of the cell; the cell 
extends and divides into two equal dorsal and ventral parts, d*? and de 
(Figs. 67 and 68). The greater surface extension of d*? in Figure 67 
is due to its being spread out in a thin layer over the surface of the 
entoderm cell. 
In dis and die (Fig. 66) the spindles are also dorso-ventral in 
position, and the cells divide equatorially into equal parts, QU qe 
ad, and ds. (Figs. 67 and 68). 
In d^? and d: the spindles lie at right angles to those just described, 
and the cells divide meridionally and equally, forming des, de-, de., 
and d*-?, 
Figure 67 is a view of this region after these cleavages are finished. 
As shown in this figure, a certain amount of shifting has taken place 
during cleavage, by which the cell d™ has been excluded from its share 
in the boundary of the blastopore. As the cells divide, they withdraw 
their interior parts and extend in the direction of the spindle, as has 
been minutely described for other cleavages; in this way the dorso- 
ventral extent of quadrant D has been greatly increased. Asa result 
the blastopore has been nearly closed (Fig. 63), and at the opposite 
end the animal pole has been pushed beyond the micromere end of the 
egg to its anterior side (Fig. 65). 
Thus far the six larger left hand cells have divided, leaving six smaller 
cells (at the right and at the dorsal pole) undivided (Fig. 67). 
Next, as shown in this figure, the cells d" and dis form „spindles and 
divide. Each cleaves in the same manner as its larger companion cell 
has done, d"? equatorially, dis meridionally. The products, dee, dee, 
d8, and de, are shown in Figure 68 (compare Diagram IV.). 
Next de cleaves equatorially, like its companion cells d"? and dine. 
In d^? the division is unequal, the ventral product, d, being much 
the smaller (Fig. 68). 
