JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 23 
axis of the preceding spindle. An optical section of the egg along its 
chief axis, showing the asters in the cells B® and Ds, is given in Plate 
2, Fig. 10. Spindles are formed in all four cells nearly or quite in 
the position indicated by the direction of separation of the asters. 
The tendency of the karyokinetie processes in the posterior half of 
the egg to gain upon those in the anterior half, shown during the last 
division, is continued and accelerated. Spindles appear in C? and Ds, 
while the nuclei in A? and B? are still spherical and have distinct mem- 
branes. Figure 9 gives a view of this stage from the right side; the 
large spherical nucleus of B? is represented by a broken outline. The 
spindle in C? has a dorso-ventral direction, and its middle coincides with 
the middle of the length of the cell; the two asters are of equal size. 
In D’ the spindle is nearer the dorsal side of the egg, and is inclined, 
passing from dorsal and anterior to ventral and posterior. The ventral 
aster is the larger. 
Cleavage takes place first in the larger cell D, separating a large 
ventral blastomere, d, from a smaller dorsal one, d. At the same 
time the two cells (considered as a whole) elongate dorso-ventrally. 
In so doing, the ventral blastomere, d*!, remains nearly stationary, 
while d *? moves in the direction of the animal pole of the egg. (Com- 
pare Figure 12, a sagittal section of a five-cell stage, with Figure 
10, the corresponding section of a four-cell stage, observing the posi- 
tion of the cells in relation to the general form of the egg.) As a 
result of this, the dorsal end of the cell B, and, to a less degree, the 
ends of A? and , are displaced in the same direction; that is, the 
whole animal pole moves toward the micromere end of the egg. At 
the same time the cells AP, B°, and C? are slightly compressed dorso- 
ventrally. This is the beginning of that peculiar rotation of the blasto- 
meres in the eggs of Rotifera, described by Zelinka (91) and others, 
which eventually results in the process of gastrulation. 
C? divides next, the cleavage being equal; the products are c* 
End gms 
Before the cleavage is finished in D? and Cs, spindles have been 
formed in B° and AP, division taking place in them in the order named. 
1 
The cleavage is equal, as in O°. 
The order of cleavage, then, for the four cells, is as follows: D, €, B, A. 
This rhythm reappears in later cleavages. 
The third cleavage is therefore equatorial, dividing the egg into two 
layers of four cells each. The ventral cells are - d, the dorsal cells 
d d. The egg is still slightly unsymmetrical. 
