90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
to the first cleavage plane, and passes through the animal pole of the egg 
(Figs. 29, 30). 
The spindle in the yolk-cell cd? is eccentric in position, lying nearer 
the animal pole of the egg, and near the centre of the protoplasmic 
mass; it is nearly perpendicular to the chief axis (Fig. 29). As cleavage 
progresses the spindle becomes inclined so that one end dips into the 
yolk-mass, which lies at the vegetative pole of the yolk-cell (Figs. 31 and 
99). From the point of view of a miniature observer occupying the 
chief axis of the ovum with his head directed toward the animal pole, the 
left end of the spindle is the one that is nearer the animal pole, that is, 
the spindle is leotropically oblique. Usually the spindle makes an 
angle of about 30° or 40° with the chief axis. 
The yolk-cell cd? cleaves unequally, and the cleavage plane may be 
considered a modified meridional one. The cleavage planes of the ‘pro- 
toplasmic ” cell ab? and of the yolk-cell meet in a line which passes 
through the animal pole, but does not coincide with the chief axis; it 
makes with this axis an angle of about 45°. To our imaginary observer 
the resulting smaller cell (¢*) lies to the left of and above the larger or 
yolk-cell d® (Fig. 31), and also this cell lies above the anterior cell 0°. 
The cell c® is the second micromere which is separated from the yolk. 
At the close of the second cleavage a general tendency towards a leo- 
tropic arrangement of the cells is noticed (Figs. 32-34, 100-102). This 
arrangement in the case of the posterior cells (c%, d®) is apparently the 
result of the oblique position of the spindle in the yolk-cell cd. When- 
ever the anterior cell a6? (first micromere) divides in advance of the yolk- 
cell ed’, there is no suggestion of a leotropic arrangement either in its 
spindle or in the position of the resulting cells (a%, 0°, Fig. 99); but 
after cleavage of the yolk-cell, the right anterior cell 5° is depressed by 
the higher lying cell c®. This change can be seen in the living ovum as 
the cleavage of the yolk-cell ed? progresses. 
~ Soon after the completion of the second cleavage the four cells tend 
to become rounded, and adjustments of position occur. Figures 32-35 
aud 102, 103 represent the arrangements which are usually seen, and 
in all of them a definite plan can be recognized. The axis of the future 
embryo can now be described as passing through the nuclei of the an- 
terior cell, 6%, and of the yolk-cell, d® (Fig. 31). The anterior cell, 0°, 
always comes to lie nearer the vegetative pole than the cells a* and c’, 
and it is usually more or less covered on the animal side by one or both 
of these cells (Figs. 34, 35). After examining the eight-cell stage, in 
which the bilateral symmetry is distinctly marked, it will be seen that 
