104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
formed to the beginning of the transformation of the nucleus and asters 
into the spindle figure — so that the relative size of the nucleus in the 
same cell from different eggs gives a measure of its relative age. 
An illustration will make clear the method used. Perhaps'the most 
difficult problem was presented by the movements of the asters and 
nuclei in the two large ventral cells of the quadrant D, at the fich and 
sixth cleavages. These cells are d°- and dë? before the fifth cleavage ; 
de and d*? after the fifth cleavage. Figure 31 (Plate 4) shows the 
position of the asters in all the ceils of this quadrant in the resting 
sixteen-cell stage. Figure 32 shows the nuclei in quadrant B in the 
same egg, as seen in a longitudinal section. In the egg shown in Fig- 
ure 33 the spindles are already completed in quadrant D, indicating 
that this egg is older than that shown in Figures 31 and 32. Figure 
35 (Plate 5), a ventral view of this stage, shows that the nuclei in a5. 
and 0^! are much larger than the nuclei in the corresponding cells of 
Figures 30 and 31, — which likewise indicates that this is an older stage 
than is represented in the latter figures. Figure 35 shows the exact 
antero-posterior position of the spindle in d*, while in the other quad- 
rants the nuclei are shown to be still in a resting condition. 
Figures 37 and 38 together show all the cells of this quadrant during 
or just after division, the four dorsal cells being still connected by inter- 
zonal filaments. It is therefore a stage later than that shown in Fig- 
ures 33 and 35. This is in agreement with the larger size of the nuclei 
in quadrant A, Figure 37, as compared with those of quadrant 4 in 
Figure 33, and also with the presence of a spindle in 4. In this egg 
the nucleus in d^! (Fig. 38) has resumed its nearly spherical form, and 
at, the side of the cell where the anterior end of the spindle was located 
(Fig. 35) is a small vesicle, d*? (Fig. 38). This represents one product 
of the division. 
In d^? at this stage (Figs. 37 and 38). the single aster is slightly 
extended in a direction which is oblique to both the longer and shorter 
axes of the cell. 
The spindles in the cells of quadrants A, B, and C in Figures 39-43 
(all from the same egg) prove that this is a later stage than that last 
considered. In this egg the line joining the two asters in d°? (Fig. 42) 
is oblique to the antero-posterior plane of the embryo, and nearly dorso- 
ventral. 
In the egg shown in Figures 43-46, division is completed in some of 
the cells of quadrants A, B, and C, proving this to be a later stage than 
those shown in Figures 37-42. In this egg the two asters in d^? (Fig. 
