The Questions of Developmental Biology

embryos can produce these cells, but they appear unable to organize them into functional tissues
and organs. Tissues of the digestive tract can be found, but they are not connected; muscle cells
are scattered around the lobeless larva, but are not organized into a functional muscle tissue.
Thus, the developmental functions of the polar lobe may be very complex.
Fate map of Ilyanassa obsoleta
As detailed above, fate maps of the snail Ilyanassa have been made means of ablation
experiments. More recently, Joanne Render (1997) has constructed a more detailed fate map by
injecting specific micromeres with beads containing the fluorescent dye Lucifer Yellow.
The fluorescence is
maintained over the period of
embryogenesis and can be seen in
the larval tissue derived from the
injected cells. Figure 8.35 shows the
new fate map of Ilyanassa obsoleta.
The second quartet micromeres (2ad)
generally contribute to the shellforming
mantle, the velum, the
mouth, and the heart. The third
quartet micromeres (3a-d) generate
large regions of the foot, velum,
esophagus, and heart. The 4d cell,
the mesentoblast, contributes to the
larval kidney, heart, retractor
muscles, and intestine.
Gastrulation in Snails
The snail stereoblastula is
relatively small, and its cell fates
have already been determined by
the D series of macromeres.
Gastrulation is accomplished
primarily by epiboly, wherein the
micromeres at the animal cap
multiply and "overgrow" the vegetal
macromeres. Eventually, the
micromeres will cover the entire
embryo, leaving a small slit at the
vegetal pole (Figure 8.36; Collier
1997).