82 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
which occur two to three hours later. Sections of ova which were fixed 
at intervals during the first hour after the close of the first cleavage 
show that the above mentioned movement of protoplasm and yolk occurs 
at about the time when the spindle and asters have disappeared (Fig. 
27). These facts suggest that the spindle and asters may have in some 
way inhibited the movement of the yolk in its return to its orig- 
inal position at the vegetative pole of the chief axis, out of which it 
appears to have been forced during the rotation of the dividing egg. 
The relative positions of spindle, protoplasmic mass and yolk, as shown 
in Figures 22-27, seem to lend support to this suggestion. The spindle 
and astral radiations appear to be arranged so as to hold the cell-sub- 
stances in the same relative positions which they occupied before the 
cleavage (Figs. 7, 22); with the disappearance of the spindle and asters 
the mass of protoplasm apparently became free to move toward the 
animal pole, while the yolk was moved to the vegetative pole (Plate 1, 
Fig. 16; Plate 3, Fig. 27). It seems that the formative and nutritive 
materials after having been displaced return to their respective poles 
of the egg as soon as the displacing and inhibiting cause isremoved. In 
this case the tendency to return to the original polar relations seems 
to be related to the phenomenon of cell-polarity, the causes of which 
are thus far hidden. 
Throughout cleavage the mass of protoplasm in the yolk-cell re- 
mains at the animal pole of the egg, which is marked by the second 
polar cell, and the successive blastomeres formed by the unequal division 
of the yolk-cell are cut off as near the animal pole as is consistent with 
the position of previously formed cells. 
Conklin (97) has pointed out for the egg of the gasteropod Crepidula 
a tendency of the protoplasmic mass in the macromeres to remain near 
the animal pole, while successive ectomeres are cut off as near that pole 
as the position of previously formed cells will allow. The condition in 
the egg of Lepas furnishes a parallel case, and the return of the pro- 
toplasmic mass to the polar position after displacement in the first cleav- 
age indicates a strong tendency towards adherence to the original 
polarity of the unsegmented ovum. 
The rotation of the dividing ovum appears to be dependent upon the 
cleavage processes, and capable of an explanation along mechanical 
lines. The cleavage furrow arises in an almost longitudinal position, 
passing through the animal pole (Plate 1, Fig. 8). As the furrow 
deepens, the forming cells tend to become spheroidal and hence to 
lengthen the axis of the ovum perpendicular to the plane of cleavage 
