The Questions of Developmental Biology

to the right. Cytoplasm from sinistrally coiling snails did not affect the right-coiling embryos.
These findings confirmed that the wild-type mothers were placing a factor into their eggs that was
absent or defective in the dd mothers.
The polar lobe: cell determination and axis formation
Molluscs provide some of the most impressive examples of mosaic development, in
which the blastomeres are specified autonomously, and of cytoplasmic localization, wherein
morphogenetic determinants are placed in a specific region of the oocyte (see Chapter 3). Mosaic
development is widespread throughout the animal kingdom, especially in protostomal organisms
such as annelids, nematodes, and molluscs, all of which initiate gastrulation at the future anterior
end after only a few cell divisions. Moreover, the cytoplasmic factors responsible for
specification are actively moved to one pole of the cell so that a blastomere containing these
factors can restrict their transmission to only one of its two daughter cells. The fate of the two
daughter cells is thus changed by which one of them gets the morphogenetic determinant.
E. B. Wilson demonstrated that mosaic development characterizes the early snail embryo
(see Figure 3.7). He also was able to demonstrate that such development is predicated on the
segregation of specific morphogenetic determinants into specific blastomeres. .
M
Certain spirally cleaving embryos (mostly in the mollusc and annelid phyla)
. extrude a bulb of cytoplasm immediately before first cleavage (Figure 8.31).
This protrusion is called
the polar lobe.
In certain species of
snails, the region
uniting the polar lobe to
the rest of the egg
becomes a fine tube.
The first cleavage splits
the
zygote
asymmetrically, so that
the polar lobe is
connected only to the
CD blastomere.
In several species, nearly one-third of the total cytoplasmic volume is present in this
anucleate lobe, giving it the appearance of another cell. This three-lobed structure is often
referred to as the trefoil-stage embryo (Figure 8.32). The CD blastomere then absorbs the polar
lobe material, but extrudes it again prior to second cleavage. After this division, the polar lobe is
attached only to the D blastomere, which absorbs its material. Thereafter, no polar lobe is formed.
Wilson (1904) showed that if
one removes the polar lobe at the trefoil