GASTRULATION AND MESODERM MORPHOGENESIS IN THE WHITE STURGEON

This study presents a detailed description of gastrulation in the whit e sturgeon, Acipenser transmontanus, using scanning electron microscop y, histology, and time-lapse filming and video microscopy. This morpho logical analysis describes the similarity of gastrula structure in the sturgeon and the amphibian Xenopus laevis, and suggests that the spec ies share many developmental mechanisms. It also identifies important differences, such as the equatorial dorsal lip in sturgeon, and provid es a basis for interpreting experiments that test the effect of these differences on gastrulation. The onset of gastrulation in the sturgeon is marked by the appearance of a blastoporal equatorial pigment line that forms as the apices of bottle cells contract and concentrate surf ace pigment granules. Bottle cell formation at the blastopore lip and involution of surface material through the blastopore are strikingly s imilar to the equivalent processes in amphibian embryos. As gastrulati on continues, a distinct cleft of Brachet forms between pre-involution and post-involution material. Following involution, the prospective a xial mesoderm located on the dorsal surface of the late blastula (Ball ard and Ginsburg: J. Exp. Zool., 213:69-103, 1980) ingresses from a ce ntral zone in the posterior archenteron roof surface in a process that is unlike any in Xenopus, but resembles events in other amphibians (P urcell, 1992; Smith: Dev. Biol., 98:250-254,1983; King: Biol. Bull., 4 :287-300, 1903). The detailed comparison of similarities and differenc es in gastrulation in different vertebrate lineages yields insights in to the function and versatility of common developmental mechanisms.