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Fig. 2. Development of the embryonic skeleton of euechinoid sea urchins.
(A) A living sea urchin embryo (Lytechinus variegatus) at the
late gastrula stage. The PMCs adopt a characteristic ring-like pattern within
the blastocoel and secrete two skeletal rudiments (arrowheads). (B) A
living pluteus larva (L. variegatus) viewed with partially crossed
polarizers (image courtesy of Dr Rachel Fink, Mount Holyoke College). The
mineralized skeleton (arrowhead) is birefringent. (C) Scanning electron
micrograph of the late embryonic skeleton (Dendraster excentricus),
with all cellular material removed.