. The Biological bulletin. Biology; Zoology; Marine biology. 40 p. SCHATT AND FERAL even in cidaroids with abbreviated development (Parks et al, 1989; 18 h after fertilization in Phyllacanthus par- vispinus.) In other species, hatching occurs at the begin- ning of gastrulation (10 h after fertilization in Peronella lesueuri—Mortensen, 1921; 15-16 hours after fertilization in Heliocidaris erythwgramma—WilUiams and Ander- son, 1975). In Abatus cordatus, hatching occurs at the end of gastrulation, ca. 65-70 days after fertilization. Species that hatch at the end of gastrulation or even late
. The Biological bulletin. Biology; Zoology; Marine biology. 40 p. SCHATT AND FERAL even in cidaroids with abbreviated development (Parks et al, 1989; 18 h after fertilization in Phyllacanthus par- vispinus.) In other species, hatching occurs at the begin- ning of gastrulation (10 h after fertilization in Peronella lesueuri—Mortensen, 1921; 15-16 hours after fertilization in Heliocidaris erythwgramma—WilUiams and Ander- son, 1975). In Abatus cordatus, hatching occurs at the end of gastrulation, ca. 65-70 days after fertilization. Species that hatch at the end of gastrulation or even later are found among the asteroids. In Patiriella regularis (indirect development), hatching by rupture of the fertilization membrane occurs 25 h after fertilization, and gastrulae become free-swimming feeding larvae (Byrne and Barker, 1991). In Asterina gibbosa (abbreviated development), hatching occurs 85 to 96 h after fertilization and produces a thoroughly transformed brachiolaria that rapidly adheres to the substratum (Marthy, 1980). In both cases, blastula and gastrula are hoUow (Byrne and Barker, 1991; Ludwig, 1882). A hypothetical new mode of gastrulation produces a pre-adult: perigastrulation Gastrulation in echinoderms involves two simultaneous but relatively independent processes, invagination to form the archenteron and migration of the primary mesen- chyme cells. As the archenteron approaches the wall of the animal region, pseudopodia-forming cells become disengaged from the archenteron and form a mass of sec- ondary mesenchyme cells that spread over the inner sur- face of the adjacent ectoderm (see Burke, 1990; Burke et al., 1991, for the mechanism). Modifications in gastru- lation have been an important component of evolution in echinoderms and other animals (Wray and Raff, 1991b; Raff, 1992). Cell movements that occur during gastrula- tion establish the topological relationships between cells of the primary germ layers, setting up all the future in- ductions be
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