80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



accurately interpreted from observations on the opaque living egg, but 

 sections of ova killed at various stages in the cleavage show some in- 

 teresting conditions. About the time when the pronuclei come into 

 contact, two clear areas are often seen near the pronuclei, as shown in 

 Figure 20 (Plate 2), but frequently in a plane more nearly transverse 

 than that in which they are shown in the figure cited. In the same 

 positions well-defined asters later make their appearance, and the first 

 cleavage spindle begins to form with its axis oblique to that of the vitel- 

 line membrane (Fig. 21). In many cases the spindle begins to form in 

 a plane almost perpendicular to the long axis of the ovum. This is true 

 particularly in L. fascicularis (compare Plate 11, Fig. 98). 



In the metaphase of the mitosis the spindle is usually oblique to the 

 long axis of the ovum (Fig. 22) ; sometimes it is almost transverse 

 (Fig. 98), but never parallel to the long axis. In L. fascicularis it is 

 most frequently perpendicular to the chief axis, as shown in Figure 

 98. In L. anatifera the spindle is usually almost as long as the trans- 

 verse axis of the ovum. The astral radiations are very distinct, and 

 appear to be continuous with the general protoplasmic reticulum of the 

 cell (Fig. 22). In the stage of the living ovum corresponding to this 

 the yolk has taken an eccentric position at the vegetative pole (Fig. 7). 

 The relation seen to exist between the yolk and the aster nearest the 

 vegetative pole (Fig. 22) suggests that the movement of the yolk to the 

 eccentric position has some relation to the formation of the aster, for 

 it is during the development of that structure that the yolk moves to 

 the eccentric position. 



In the next stage figured, an early anaphase (Plate 3, Fig. 23), the 

 spindle is still oblique and the cleavage furrow has not begun to form. 

 The chromosomes have separated along a plane which is usually inclined 

 to the plane in which the cleavage furrow later appears. This stage 

 corresponds to a stage of the living ovum which is slightly later than 

 that represented in Figure 7. 



Figure 24 represents a stage in the anaphase after the cleavage fur- 

 row has become well developed, and the dividing ovum has begun to 

 rotate. This is the condition in stages of the living egg corresponding 

 to those shown in Figures 10-13. The central part of the spindle is 

 almost perpendicular to the plane of cleavage, but there is a distinct 

 bend in the spindle near either end. These bends may be regarded as 

 evidence of torsion. Comparing Figures 23 and 24, it appears that 

 during division there has been some shifting of the egg substance with 

 reference to the spindle, which is at first somewhat oblicpie to the plane 



