The eggs of echinoderms have relatively little yolk and the yolk is evenly distributed throughout the egg.
Morula:
Cleavages and holoblastic and result in smaller blastomeres. In just few hours the morula is produced. As cell division continuous, cells pull away from the interior of the embryo. A fluid filled cavity the blastocoel forms and the cells form single layer around the cavity. The embryo is now a hollow sphere called blastula. In sea urchins development through the blastula stages takes place within the fertilization membrane. When the cells of the blastula develop cilia, the blastula breaks out of the fertilization membrane and begins to swim. Late in the blastula stage, groups of cells break free of the animal and of the embryo and position themselves within the blastocoel. These cells called primary mesenchyme will form skeletal elements (spicules) of the embryo.
FIG will be attached, photo copy
Gastrulation:
First sign of gastrulation is the imagination of cells at a blastopore in the vegetal half of the embryo. During imagination, an embryonic gut, the archentron elongates and reduces the size of the blastocoel.
During gestrulation the embryo also begins to lengthen and assumes a pyramidal shape. Larvae have an anterior end with head and tail at posterior end. The shape changes that occur during gastrulation establish the anteroposterior axis of the embryo.
Morphogenesis:
Finally coelom forms from out pockets of the archentrons and the gut breaks through the anterior body wall. The opening thus produced is the mouth.
The cell movements that begin in gastrulation result from groups of cells changing their shapes simultaneously. Contractile micro filaments mediate these shape changes and form embryo. In sea urchin these changes produce a pluteus larva that swims freely in the sea and feeds on even smaller plants and animals.
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