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So, we had established through other experiments that migrating primordium requires a receptor
at the rear that acts as a vacuum cleaner to locally reduce the guidance molecule
-- to suck it up -- and, therefore, to generate a directional cue. Here we can see a mutant
tissue that lacks this vacuum cleaner receptor -- it's labelled in red -- and normally this
migrating tissue will not move due to the absence of this vacuum cleaner at the rear.
However, the expression of the vacuum cleaner receptor within the nerve now drives a new
type of migration. Here, the primordium's behaviour is dependent on the position of
the nerve, because the nerve is providing the vacuum cleaner. Now, what we find in this
particular case is, when the nerve goes too far forward and actually comes through the
tissue -- it has some degree of freedom -- you'll find that the tissue's movement arrests, because
now the vacuum cleaner has come too far ahead of the tissue, we think. However, when the
nerve then falls back you can see that the migration of the tissue is much more efficient,
and that's because now we think the vacuum cleaner molecule is in a position that is
appropriate to generate this path.