On April 11, 2010 I observed the early spicule formation in one-day-old embryos of the sand dollar, Dendraster excentricus. The embryos hatched from their fertilization envelopes and have nearly completed gastrulation, you can see the invaginated primary gut (archenteron) almost touching the roof of the blastocoel (the space between the outer and inner layer of cells in the embryo). Particularly noticeable were the progeny of the micromeres, which are the small cells at the vegetal pole at the 16 cell stage. These cell give rise to the primary mesenchyme cells, which ingress into the blastocoel, and secrete the calcareous spicules, which form the larval skeleton.
By using cross-polarized light (one polarizing filter placed above the specimen and one below), I was able to visualize the spicules in stark contrast. With this technique, the light that passes through the first polarizer is blocked by the second, and the only structures that remain bright are those that rotate the plane of polarized light e.g. various crystalline structures - in this case skeletal spicules, composed of calcium carbonate. This highlights the spicules on a dark background. The first two spicules in urchin larvae form at the base of the archenteron, one on each side, where the primary mesenchyme cells are concentrated. The initial spicule is tri-radiate. The three branches grow and form the postoral and antero-lateral arm rods, and the body rod of the pluteus larva.