Chapter Three – ANIMALIA

Publisher Summary The chapter defines animals as heterotrophic, diploid, multicellular organisms that usually develop from embryos. The blastula, a multicellular embryo that forms from the diploid zygote produced by fertilization of a large haploid egg by a smaller haploid sperm, is unique to animals. Animals are called anisogametes because animal gametes—the egg and sperm—differ in size. The diploid zygote produced by fertilization divides by mitotic cell divisions often a solid ball of cells that hollows out to becomea blastula. In many animals, the blastula develops an opening called the blastopore, which is the opening to the developing digestive tract. The blastopore will be the site of the mouth in some phyla or the anus in others. In some phyla young animals show neither of these two patterns. Some animals with spiral cleavage produce a blastula (stereoblastula) that is a solid ball of cells—their affinities remain unclear until more is known. Cephalopod molluscs, which have much yolk, lack blastocoels (embryonic cavities). Cell differentiation and cell migration transform blastula into a gastrula. Embryos have dead-end or tubular indentations that become the embryonic digestive tract in most.