The earliest events in development of the brittle star Ophiopholis aculeata proceed very similarly to an urchin or seastar, but with a curious twist. 

Fertilization results in elevation of an extracellular envelope, plus a hyaline layer like an urchin egg would make but much thicker; seastar eggs usually make little or no hyaline. 

As in seastars, female meiosis completes after spawning; the polar bodies, universal hallmarks of the animal pole, are extruded either before or just after fertilization.  In urchins, unusually, oocytes complete meiosis in the ovary, so polar bodies are rarely seen on spawned eggs.

Cleavage eventually creates a hollow blastula; as in a starfish, all divisions are equal.  In an urchin, the fourth cleavage is highly asymmetrical in the vegetal hemisphere, creating micromeres which give rise to the larval skeleton.  Brittle stars also have a larval skeleton, but they don't make micromeres.

The twist involves the zygote spindle, the first cleavage plane, and the partitioning of cell fate determining factors within the egg.  In this video, the approximate positions of spindle poles are marked by asterisks.  The spindle initially lines up balanced directly beneath the polar bodies, as if to divide perpendicular to the animal-vegetal (AV) axis like a good echinoderm embryo should.  But then the spindle rotates slides to the side and the first division occurs at nearly a 45 degree angle to the AV axis.

This shift in the cleavage plane implies a departure from the echinoderm stereotype.  In most other indirect-developing echinoderm embryos, when the two daughter cells of first cleavage are separated, they both develop into small but complete larvae.  The two cells each contain the same stuff, all of the essential information to trigger development of a complete larva; likewise at the four-cell stage.  The contents of the egg may be differentiated along the AV axis, but since the first two cleavage planes are parallel to the AV axis, the resulting cells are the same. 

But in Ophiopholis, all of the vegetal domain along with whatever information it contains gets incorporated into a single daughter cell at the first (and again at second) cleavage.  Thus, each cell resulting from the first two divisions contains different subsets of the oocyte's animal-vegetal axis.  Alex Primus, who first noticed the spindle rotation in this species, showed that when Ophiopholis blastomeres are separated at the two-cell stage, one of them gives rise to a small but complete larva while the other results in an abnormal larva that has a tiny gut and a reduced larval skeleton.*

This reveals that there is indeed some kind of consequential difference in the information contained in different parts of the egg.  Developmental biologists would say there is a cytoplasmic determinant for endoderm and mesoderm specification associated with the vegetal pole. 

Why do these embryos bother to adjust the cleavage plane?  Throughout the animal kingdom, there are numerous instances in which some group of animals has evolved a mechanism to precociously determine cell fates by segregation of determinants, but in most cases, including this one, it's not obvious why this should be adaptive.

— text by George von Dassow & Katie Bennett

* A. E. Primus (2005) Developmental Biology 283:294–309