JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 55 
considerable shiftings which take place. As seen in Figure 65, the 
animal pole lies at the anterior margin of the cell ds. 
Before the eighth cleavage takes place, the blastopore has become 
closed and its anterior margin has begun to pass into the two-layer 
condition, as previously described. The larger cell, ., divides before 
its mate, by a spindle at right angles to the previous spindle. The 
cleavage is equal, forming the two large right and left cells % and 
d*? shown in Plate 10, Fig. 80. The plane separating these cells 
coincides with that separating the quadrants A and Z on the anterior 
side of the egg, and also with that separating the two cells deu and 
de on the posterior side (Plate 8, Fig. 68). As this plane also 
passes through the animal pole and the blastopore, it divides the egg 
into symmetrical halves, and is the median dorso-ventral or sagittal 
plane of the embryo. 
Later the small cell, d*:?, develops a spindle in the same direction as 
the spindle of the seventh cleavage, in the shorter axis of the cell, and 
divides into two very unequal cells. The anterior or ventral cell, d, 
is a minute vesicle, whereas the dorsal or posterior blastomere, d?*, is 
scarcely smaller than the mother cell. The process of budding off this 
small cell is shown in Plate 10, Figure 80. "The vesicle lies between 
the two large cells d™! and d., and, like the minute cells d*? and d"? 
g lost among the many 
5 
can be traced but a short distance, soon becomin 
cells by which it is surrounded. 
In the ninth cleavage, spindles are formed in the cells d? and d?? in 
the position foreshadowed by the asters in Figure 80, — that is, antero- 
posterior, and at right angles to the preceding spindles, — and the cells 
divide equally, forming the four cells d., d'™?, 0.8. and de.. Figures 
76 and 81 show the entoderm at the close of these divisions — the 
nuclei of the cells in question being still connected in -pairs by inter- 
zonal filaments. The blastopore is now present as a distinct notch ; its 
anterior or dorsal lip has become two-layered, owing to the folding 
inward of the cells of the anterior quadrants. The animal pole (pol. 
anm., Vig. 76) has moved a considerable distance on to the previously 
anterior surface, lying still at the anterior margin of the entoderm cell 
d**, As the side view (Fig. 76) shows, a frontal plane carried through 
the long axis of the egg at this stage would cut the nuclei of all the 
entoderm cells, as realized in the frontal section, Figure 81 (Plate 10). 
As soon as the cells, after the cleavage process is entirely finished, 
have lost their strong tendeney to maintain a form as nearly spherical as 
possible, a sudden and considerable change of relative position takes 
