What is Echinoderms Embryology
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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