The Questions of Developmental Biology

These rapid and invariant cell
cleavages last through the ninth or tenth
cell division, depending upon the species.
After that time, there is a mid-blastula
transition, when the synchrony of cell
division ends, new genes become
expressed, and many of the nondividing cells develop cilia on their outer surfaces; Masuda and
Sato 1984). The ciliated blastula begins to rotate within the fertilization envelope. Soon
afterward, differences are seen in the cells. The cells at the vegetal pole of the blastula begin to
thicken, forming a vegetal plate. The cells of the animal half synthesize and secrete a hatching
enzyme that digests the fertilization envelope (Lepage et al. 1992; Figure 8.11C). The embryo is
now a free-swimming hatched blastula.
Fate maps and the determination of sea urchin blastomeres
Cell fate determination
The fate map of the sea urchin embryo was originally created by observing each of the cell layers
and what its descendants became. More recent investigations have refined these maps by
following the fates of individual cells injected with fluorescent dyes such as diI (see Chapter 1).
These studies have shown that by the 60-cell stage, most
of the embryonic cell fates are specified, but that the cells are not
irreversibly committed.
In other words, particular blastomeres
consistently produce the same cell types in each
embryo, but these cells remain pluripotent and
can give rise to other cell types if experimentally
placed in a different part of the embryo.