into two equal cells (C). First, the nucleus of the stem-cell and its
central body divide into two halves. These recede from and repel each
other, and act as centres of attraction on the surrounding protoplasm;
in consequence of this, the protoplasm is constricted by a circular
furrow, and, in turn, divides into two halves. Each of the two
segmentation-cells thus produced splits in the same way into two equal
cells. The four segmentation-cells (grand-daughters of the stem-cell)
lie in one plane. Now, however, each of them subdivides into two equal
halves, the cleavage of the nucleus again preceding that of the
surrounding protoplasm. The eight cells which thus arise break into
sixteen, these into thirty-two, and then (each being constantly
halved) into sixty-four, 128, and so on.* (* The number of
segmentation-cells thus produced increases geometrically in the
original gastrulation, or the purest palingenetic form of cleavage.
However, in different animals the number reaches a different height,
so that the morula, and also the blastula, may consist sometimes of
thirty-two, sometimes of sixty-four, and sometimes of 128, or more,
cells.) The final result of this repeated cleavage is the formation of
a globular cluster of similar segmentation-cells, which we call the
mulberry-formation or morula. The cells are thickly pressed together
like the parts of a mulberry or blackberry, and this gives a lumpy
appearance to the surface of the sphere (Figure E).* (* The
segmentation-cells which make up the morula after the close of the
palingenetic cleavage seem usually to be quite similar, and to present
no differences as to size, form, and composition. That, however, does
not prevent them from differentiating into animal and vegetative
cells, even during the cleavage.)

When the cleavage is thus ended, the mulberry-like mass changes into a
hollow globular sphere. Watery fluid or jelly gathers inside the