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| Without a coelom, as in flatworms and proboscis worms. |
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| The main cavity of an embryo in the gastrula stage; it is lined with endoderm and represents the future digestive cavity. |
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| External opening of the archenteron in the gastrula. |
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| Early embryological stage of many animals; consists of a hollow mass of cells. |
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| Process of nuclear and cell division in animals zygote. |
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| The body cavity in triploblastic animals, lined with mesodermal peritoneum. |
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| Animals that possess a coelom. |
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| Any member of the phyla (Chordata, Hemichordata, Echinodermata, Chaetognatha) in which the anus appears first, developing at or near the blastopore, cleavage is radial and indeterminate, and the mesoderm and coelom form from outgrowths of the primitive gut. |
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| Outer layer of cells of an early embryo; one of the germ layers, also sometimes used to include tissues derived from ectoderm. |
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| Innermost germ layer of an embryo, forming the primitive gut; also may refer to tissues derived from endoderm. |
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| Any of the animals with "true" coelom, which is characterized by a body cavity completely lined with a peritoneum that attaches to organs. |
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| The membrane that forms on the inner surface of the vitelline membrane of the ovum after the entry of a spermatozoon, thereby acting as a barrier to prevent additional penetration of other spermatozoa. |
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| A process in sexual reproduction that involves the union of male (sperm) and female (ovum) gametes (each with a single, haploid set of chromosomes) to produce a diploid zygote. |
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| A mature haploid sex cell; usually male and female gametes can be distinguished. An egg or sperm. |
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| Embryonic stage, usually cap- or sac- shaped, with walls of two layers of cells surrounding a cavity (archenteron) with one opening (blastopore). |
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| Process by which an early metazoan embryo becomes a gastrula acquiring first two and then three layers of cells. |
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| In an animal embryo, one of three basic layers (ectoderm, endoderm, mesoderm) from which the various organs and tissues arise in the multi-cellular animal. |
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| Any member of the lower invertebrate phyla in which the mouth appears before the anus during development, cleavage is spiral and determinate, and the coelom forms as a splitting of the mesoderm. |
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| Having a body cavity formed from a persistent blastocoel and lined with mesoderm on only one side. |
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| The third germ layer, formed in the gastrula between the ectoderm and endoderm; gives rise to connective tissues, muscle, urogenital and vascular systems, and the peritoneum. |
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| Pertaining to animals in which the embryo has three primary germ layers- ectoderm, mesoderm, and endoderm. |
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