20 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Callidina the first cleavage spindle is oblique to the long axis of the egg, 
therefore not in agreement with Hertwig's law ; but immediately after 
division is finished, a movement of the egg contents takes place in 
such a way that the two cells occupy the same relative position as in 
Asplanchna, — such a position, therefore, as is demanded by Berthold’s 
theory of least surfaces. It thus appears that in Callidina the direc- 
tion of division itself is determined neither by the principle of Berthold 
nor that of Hertwig, but that the later arrangement of the cells might 
be held to be due to the action of Berthold’s principle. It is somewhat 
curious that the exact arrangement produced in Callidina by shifting 
should in Asplanchna result at once from the position of the spindle at 
the time of cleavage. 
No cause can be assigned, from the visible structure of the egg, for 
the inequality of the cleavage. The yolk granules are distributed uni- 
formly throughout the egg, seeming no more abundant in the large 
than in the small cell. 
Second Cleavage. 
As a result of the first cleavage, the egg is now composed of two un- 
equal blastomeres, an anterior, AB’, and a posterior, CD" (Figs. 5 and 6). 
In the smaller blastomere, as previously stated, the aster has already 
divided and the two parts are separating at the time when the first 
cleavage plane passes through the cytoplasm (Fig. 4). The line along 
which they move apart is perpendicular to the axis of the first cleavage 
spindle, aud also at right angles to a line connecting the polar cell with 
the centre of the egg. The forming spindle is thus parallel to the 
lateral axis of the embryo and consequently perpendicular to its dorso- 
ventral axis. The two asters take up their positions on opposite sides 
of the nucleus, and the axis of the resulting spindle has a direction 
parallel to the line joining the asters at their first separation (Wigs. 5 
and 6). Meanwhile the nucleus has steadily inereased in size, up to the 
time when it participates in the formation of the spindle. 
In the larger cell, CD’, the order of procedure is different. The 
nucleus begins to enlarge, as in the smaller blastomere, but the aster 
does not at once divide. The nucleus and aster together begin to 
migrate to the right. At the same time the aster comes to lie farther 
to the right than the nucleus, either because the two rotate on a com- 
mon axis, or because the aster, moving faster, creeps around the nucleus 
toward the right side of it. "Thus, whatever the mothod, a condition is 
reached in which the large nucleus lies in tho right anterior angle of the 
