Editing Blastocoel (section)

=== Development of primary mesenchyme ===
Important to the sea urchin blastula is the ingression of the [[primary mesenchyme]]. After the blastula hatches from the fertilization envelope, the [[Vegetal pole|vegetal]] side of the blastula begins to flatten and thicken as a small cluster of these cells develop long, thin processes called [[filopodia]]. These cells then dissociate and ingress into the blastocoel and are called the primary mesenchyme. The cells move randomly along the inside of the blastocoel, until they become localized in the ventrolateral region of the blastocoel.<ref name="Cherr"/><ref name="Galileo"/>