JENNINGS: DEVELOPMENT OF ASPLANCHNA IIERRIOKII. 51 
place have commonly been already somewhat elongated in the direction 
of the spindle. Therefore, at the occurrence of division, the short cells, 
cleaving equatorially, have changed form greatly, becoming more elon- 
gated dorso-ventrally, while the cells which eleave meridionally have 
already been of sufficient, length to permit of the extension of the spin- 
dle without much change of form. As a result, there has been a great 
extension of the ectoderm dorso-ventrally. This produces first a com- 
plete elosuro of the blastopore. (Compare Figure 65, Plate 8, with 
Figure 76, Plate 9.) A second result, due to the larger size of the 
cells of the quadrant D, and perhaps partly also to the fact that they 
oleave first, is tho further displacement of the animal pole of the egg 
from the micromere end toward the anterior side (Plate 8, Fig. 65, 
Plate 9, Fig. 76). 
The closure of the blastopore is not sufficient to provide for the dorso- 
ventral extension brought about during the eleavage, so that, as a third 
consequence, the cells at the ventral pole of the egg, where the blasto- 
poro was previously situated, are pushed over or under one another, 
the ectoderm tending to become two-layered in this region. A compari- 
son of Figure 63 (Plate 8) with Figure 73 (Plate 9) shows the result- 
ing conditions, In the latter figure the entoderm (dt in Fig. 63) is 
entirely enclosed, and the ventral cells a't- ce", a? c., at — 013, and 
(A- have become crushed together, and several of them, as a^? and 
078, are almost hidden by surrounding cells. The beginning of the two- 
layer condition is shown in frontal section in Figure 80 (Plate 10), 
from the same egg as that shown in Figure 73 (Plate 9). 
Meanwhile other cleavages are taking place in quadrant D, leading to 
still further modifications of the structure of the egg. It is therefore 
necessary to return to a consideration of this quadrant. 
Quadrant D. — By reference to the diagram of the cells of this quad- 
rant at the end of the seventh cleavage (Diagram IV., page 48), it will 
be secn that there are now present, exclusive of the entoderm, twenty- 
three cells, arranged somewhat irregularly. Approximately the same 
stave, as it actually appears, is shown in Fignre 68 (Plate 8). 
In this egg spindles. have appeared in the large cells d* and d*™, and 
later they each become divided into two equal cells. It is worthy of 
notice that the two spindles are not parallel, but make a slight angle 
with each other. The two cells lie on opposite sides of the median 
dorso-ventral plane, so that the angle between the spindles indicates the 
beginning of a tendency to bilateral cleavage. The inclined position of 
the other spindles (in c? and c^) is indicative of tho same fact. The 
