Fig. 1.

Spatial expression of selected skeletogenesis genes in Et Alx1 expression is restricted to skeletogenic precursors throughout development; by 50 h, alx1-positive cells are seen migrating to the vegetal lateral clusters, where they will synthesize the larval skeleton. Delta is first expressed in micromere-descendants before hatching and is restricted to this lineage until late blastula stage (20 h), where it is expressed in scattered cells at the tip of the archenteron. Zygotic expression of hesC begins in a ring of cells that abut the micromere-descendants at the vegetal pole; by early gastrula, it is asymmetrically expressed in the archenteron [20 h, tip of archenteron/apical view (AV)]. Expression of ets1 begins in a few cells at the vegetal pole before hatching and expands to demarcate the whole mesodermal domain, eventually occupying the entire mesodermal bulb by early gastrula stage. Onset of zygotic tbrain expression occurs at the vegetal pole shortly after that of ets1, to which it exhibits very similar spatial expression. h, hours after fertilization; LV, lateral view; VV, vegetal view.