By Michael Hutchins (ed.)
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Additional info for Grzimek's Animal Life Encyclopedia, Vol 1: Lower Metazoans and Lesser Deuterostomes
Example text
In turn, each layer will give rise to certain tissues of the adult body. Prior to forming layers, most embryos go through a stage of minor reorganization known as blastulation, which provides the spatial framework in which the actual layering takes place. Grzimek’s Animal Life Encyclopedia Vol. 1: Lower Metazoans and Lesser Deuterostomes The vast majority of animals develop either two or three layers of cells, known as germ layers. These germ layers develop out of the dramatic reorganization of the cells that were formed during cleavage and slightly reorganized during blastulation.
This species usually shows either little outward hydrorhizal growth combined with a high rate of reproduction, or extensive hydrorhizal growth combined with a lower rate of reproduction and correspondingly greater competitive ability. Contact with another colony leads the hydrorhiza to produce an abnormally large number of stolons (shoots or runners) armed with nematocysts, which sting and kill the tissue of other hydroids. Guerrilla growth strategies have adaptive value in situations where there is relatively little space available, as on shells occupied by other hydroids, while the phalanx strategy is more advantageous for expanding the colony to shells that are inhabited by hydroids.
It begins with simple recognition among the sperm and oocytes of a given species, and concludes with the fusion of the hap18 Vol. 1: Lower Metazoans and Lesser Deuterostomes loid pronucleus of the sperm and the haploid pronucleus of the oocyte. Between these two events, there is considerable variation among the lower metazoans. In some cases, the oocytes have a covering that must be penetrated by the sperm. In most cases, the sperm must initiate the actual process of fusion of the cell membranes of the two gametes; however, it is the oocyte that generally is most active in directing the fusion and actually incorporating the sperm into its cytoplasm.