22 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
(Plate 2, Fig. 8) shows that the cleavage planes have taken the posi- 
tions foreshadowed by the arrangement of the spindles. The plane 
separating C? from D? lies to the right of the plane separating A? from 
B?; the corresponding furrows on the surface are nearer together on 
the dorsal than on the ventral side. The blastomeres resulting from 
the division of AB” are equal, whereas CD divides very unequally. 
The right derivative (C?) is much smaller than the left (D*), and is of 
approximately the same size as A’ and 3°. The blastomeres B? and 
D? are in contact along the whole distance from the dorsal to the ven- 
tral surface of the egg, while 4? and O? do not touch each other at all. 
The polar cell lies either at the junction of B°, Cs, and Ds, as shown in 
Figure 8, or sometimes at the junction of 4% B and D. The egg is 
now markedly unsymmetrical. 
It is evident from the above description that this cleavage may be 
considered as belonging to the so called spiral type. Since the left end 
of the spindle is in each cell the higher, tho cleavage is a left spiral, 
like the corresponding cleavage in Discocelis, Nereis, Limax, and indeed 
all forms with spiral cleavage except in the reversed cleavage of certain 
mollusks. This fact is striking, since the succeeding cleavages in 
Asplanchna do not belong to the spiral type. 
The relation between the axes of the embryo in later stages and the 
first two cleavage planes is as follows. The first furrow separates an 
anterior from a larger posterior portion, but the plane of separation of 
the parts bears no simple relation to the axes of the later embryo. 
(Compare Figure 8 with Figure 75, Plate 9, in which the parts derived 
from the first four cells are colored in the same manner as their parent 
cells in Figure 8, and note the great shifting.) The later sagittal plane 
of the embryo is coincident with a plane passing through the animal pole 
and the longest axis of the egy; that is, through the plane separating A’ 
from B? (Fig. 8), and dividing the larger blastomere D’ into two un- 
equal parts. The second cleavage plane therefore divides the right side 
from the left in the anterior part of the egg; but in the posterior part it 
lies entirely in the right side. It is not until the seventh cleavage that 
the division into symmetrical right and left halves takes place on the 
posterior side (Plate 7, Fig. 58) ; indeed, certain cells containing material 
for both sides of the egg remain undivided till even a later st 
go, 
Third Cleavage, 
Immediately after the second cleavage, the aster in ench of the four 
cells produced begins to extend dorso-ventrally, at right angles to the 
