Editing Blastocoel (section)

== In sea urchins ==
At the 120- cell stage, the sea urchin embryo is considered a blastula because of its developed blastocoel, which every embryonic cell surrounds and touches. Every cell is in contact with the proteinaceous fluid of the blastocoel on the inside and touches the hyaline layer on the outside. The loosely connected blastomeres are now tightly connected because of [[tight junction]]s that create a seamless epithelium that completely encircles the blastocoel.<ref name="Galileo">{{cite journal |last1=Galileo |first1=Deni S. |last2=Morrill |first2=John B. |title=Patterns of cells and extracellular material of the sea urchinLytechinus variegatus (Echinodermata; Echinoidea) embryo, from hatched blastula to late gastrula |journal=Journal of Morphology |volume=185 |issue=3 |year=1985 |pages=387–402 |doi=10.1002/jmor.1051850310 |pmid=29991195 |s2cid=51615081 }}</ref> Even as the blastomeres continue to divide, the blastula remains one-cell thick and thins out as the embryo expands outward. This is accomplished in part due to the influx of water that expands the blastocoel and pushes the cells surrounding it outwards. At this point, the cells have become specified and are ciliated on the opposite side of the blastocoel. The vegetal plate and animal hemisphere develop and secrete a hatching enzyme that digests the fertilization envelope and allows the embryo to now become a free-swimming hatched blastula.<ref name="Cherr">{{cite journal |last1=Cherr |first1=GN |last2=Summers |first2=RG |last3=Baldwin |first3=JD |last4=Morrill |first4=JB |title=Preservation and visualization of the sea urchin embryo blastocoelic extracellular matrix. |journal=Microscopy Research and Technique |date=15 June 1992 |volume=22 |issue=1 |pages=11–22 |doi=10.1002/jemt.1070220104 |pmid=1617206|s2cid=32044141 }}</ref>

=== 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"/>