At the end of gastrulation (50-65 days), part of the animal ectoderm thickens and forms two tissue layers separated by a lumen.
The main characteristics of this development are (1) incomplete cleavage that becomes holoblastic; (2) external migration of mesenchyme cells, in the perivitelline space, from the animal to the vegetal pole during gastrulation; (3) hatching that occurs at the end of the gastrulation; (4) differentiation of the vestibule as soon as the end of gastrulation is attained; and (5) production of a juvenile directly from the gastrula without any larval stage.
Gastrulation may be initiated within one of the wrinkles, as observed in Heliocidaris erythrogramma (Williams and Anderson, 1975).
In other species, hatching occurs at the beginning of gastrulation (10 h after fertilization in Peronella lesueuri - Mortensen, 1921; 15-16 hours after fertilization in Heliocidaris erythrogramma - Williams and Anderson, 1975).
A hypothetical new mode of gastrulation produces a pre-adult: perigastrulation