!e starfish is used to illustrate early development because the events of cleavage and gastrulation are especially easy to see. More complex development patterns in other organisms may be compared to this simple pattern.
Thus, we traced the movement of gration of cells from the outer layer to the archenteron cells into the archenteron wall by measuring the volume wall during starfish gastrulation.
ANIMAL DEVELOPMENT—Starfish The slide of stages in starfish development illustrates stages from unfertilized egg to gastrula— typical of echinoderms and chordates that ha. e very little yolk in the egg. Three embryonic cell layers—ectoderm, endoderm, . nd mesoderm—are established. All adult tissues. are derived from these layers. Starfish de.
The morphogenetic process called gastrulation rearranges the cells of a blastula into a three-layered (triploblastic) embryo, called a gastrula, that has a primitive gut (archenteron).
By the late gastrula stage all three germ layers are visible with ectoderm forming the outside surface, endoderm lining the archenteron, and enterocoelic mesodermal pouches located on either side of the archenteron.
As the starfish larva undergoes gastrulation, the archenteron forms by invagination of the thickened wall of the blastula at the vegetal pole. This results in the formation of a tubular sac containing the archenteron that opens to the external environment through a hole called the blastopore.
The endoderm of the archenteron will fuse with the ectoderm of the blastocoel wall. At this point gastrulation is complete, and the embryo has a functional digestive tube.
Gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. This diagram is color-coded: ectoderm, blue; endoderm, green; blastocoel (the yolk sac), yellow; and archenteron (the primary gut), purple.
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