The Questions of Developmental Biology

they begin gastrulation, making it possible to follow the fate of each cell of the blastula. When the
fates of the individual blastomeres from annelid, flatworm, and mollusc embryos were compared,
many of the same cells were seen in the same places, and their general fates were identical
(Wilson 1898). Blastulae produced by radial cleavage have no blastocoel and are called
stereoblastulae.
Figures 8.26 and 8.27 depict the cleavage of mollusc embryos. The first two cleavages
are nearly meridional, producing four large macromeres (labeled A, B, C, and D). In many
species, the blastomeres are different sizes (D being the largest), a characteristic that allows them
to be individually identified. In each successive cleavage, each macromere buds off a small
micromere at its animal pole. Each successive quartet of micromeres is displaced to the right or to
the left of its sister macromere, creating the characteristic spiral pattern. Looking down on the
embryo from the animal pole, the upper ends of the mitotic spindles appear to alternate clockwise
and counterclockwise. This causes alternate micromeres to form obliquely to the left and to the
right of their macromere. At the third cleavage, the A macromere gives rise to two daughter cells,
macromere 1A and micromere 1a. The B, C, and D cells behave similarly, producing the first
quartet of micromeres. In most species, the micromeres are to the right of their macromeres
(looking down on the animal pole). At the fourth cleavage, macromere 1A divides to form
macromere 2A and micromere 2a; and micromere 1a divides to form two more micromeres, 1a 1
and 1a 2 . Further cleavage yields blastomeres 3A and 3a from macromere 2A, and micromere 1a 2
divides to produce cells 1a 21 and 1a 22 .