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 is removed. 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. 



Conkliu ('97) has pohited 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 



