Fig. 1.

The larval CB and its embryonic precursor. (A) Sea urchin larva at 72 hpf as viewed by DIC microscopy. High magnification along the vegetal oral margin of the larva partially reveals the CB. Low magnification (inset) shows the visible portion of the CB (white box) relative to the location the blastopore (β). (B,C) Pattern of gene expression revealed by RNA in situ hybridization. The zygotic expression pattern of the regulatory gene onecut, shown here at 25 hpf, is the first to delineate the CB (as early as 24 hpf). (B) Lateral view. (C) Oral view. The internal domain surrounded by the CB consists of oral ectoderm, although this itself is divided along the animal (top)/vegetal (bottom) axis into several subdomains (Li et al., 2014).