In many animals cleavage leads to a structurally uniform blastula, a hollow ball whose wall consists of a simple layer of similar cells, the blastoderm. We call such a layer of morphologically still undifferentiated cells a blastema (F. E. Lehmann). It is characteristic for many phyla that, in the development of the germ layers and organs, such cell layers, rather than individual cells, play the main role. From the primitive blastema of the blastoderm, the components of the basic body plan arise stepwise in primitive development through morphogenetic movements and segregation; thus, a spatial ordering of the organ anlagen of the typical body plan or of the organs of a larva is completed. If the larva is really different from the mature form, as, for example, in echinoderms and amphibia, the end of a developmental stage is clearly delineated; in other cases the termination of early development is less sharp.
KeywordsAnimal Pole Vegetal Pole Paracentrotus Lividus Apical Tuft Stiff Seta
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