The Questions of Developmental Biology

lastocoel wall (Dan and Okazaki 1956; Schroeder 1981). The filopodia attach to the wall at the
junctions between the blastoderm cells and then shorten, pulling up the archenteron. Hardin
(1988) ablated the secondary mesenchyme cells with a laser, with the result that the archenteron
could elongate to only about two-thirds of the normal length. If a few secondary mesenchyme
cells were left, elongation continued,
although at a slower rate. The
secondary mesenchyme cells, then,
play an essential role in pulling the
archenteron up to the blastocoel wall
during the last phase of invagination.
But can the secondary
mesenchyme filopodia attach to any part of the blastocoel wall, or is there a specific target in the
animal hemisphere that must be present for attachment to occur? Is there a region of the
blastocoel wall that is already committed to becoming the ventral side of the larva? Studies by
Hardin and McClay (1990) show that there is a specific "target" site for the filopodia that differs
from other regions of the animal hemisphere. The filopodia extend, touch the blastocoel wall at
random sites, and then retract. However, when the filopodia contact a particular region of the
wall, they remain attached there, flatten out against this region, and pull the archenteron toward it.
When Hardin and McClay poked in the other side of the blastocoel wall so that the contacts were
made most readily with that region, the filopodia continued to extend and retract after touching it.
Only when the filopodia found their "target" did they cease these movements. If the gastrula was
constricted so that filopodia never reached the target area, the secondary mesenchyme cells
continued to explore until they eventually moved off the archenteron and found the target tissue
as freely migrating cells. There appears, then, to be a target region on what is to become the
ventral side of the larva that is recognized by the secondary mesenchyme cells, and which
positions the archenteron in the region where the mouth will form.
As the top of the archenteron meets the blastocoel wall in the target region, the secondary
mesenchyme cells disperse into the blastocoel, where they proliferate to form the mesodermal
organs (see Figure 8.17, 13.5 hours). Where the archenteron contacts the wall, a mouth is
eventually formed. The mouth fuses with the archenteron to create a continuous digestive tube.
Thus, as is characteristic of deuterostomes, the blastopore marks the position of the anus.
*They probably activate the genes necessary for producing the inducing signal, and a Notch ligand (such as Delta) may
be one of the signal's components
The Early Development of Snails
Cleavage in Snail Eggs
Spiral holoblastic cleavage is characteristic of several animal groups, including annelid
worms, some flatworms, and most molluscs. It differs from radial cleavage in numerous ways.
First, the cleavage planes are not parallel or perpendicular to the animal-vegetal axis of the egg;
rather, cleavage is at oblique angles, forming a "spiral" arrangement of daughter blastomeres.
Second, the cells touch one another at more places than do those of radially cleaving embryos. In
fact, they assume the most thermodynamically stable packing orientation, much like that of
adjacent soap bubbles. Third, spirally cleaving embryos usually undergo fewer divisions before