Biology  5/e   Raven/Johnson
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Ectoderm, mesoderm, endoderm: Ectoderm, mesoderm, and endoderm are the three primary tissue layers of all vertebrates. They are the first three tissues to form as the embryo develops and they represent the outer, middle, and inner tissue layers, respectively. All other tissues and organs are derived from these initial three.

Coelom: A coelom is the type of body cavity found in the more advanced invertebrates and the vertebrates. It is completely surrounded or bounded by mesoderm tissue. This makes it different from the pseudocoel of lower invertebrates that develops between the mesoderm and endoderm. The animal's internal organs (e.g., digestive, reproductive) develop within or around the margins of the coelom, and hang down into it, suspended by double layers of mesoderm called mesentaries.

Radial and spiral cleavage: Radial and spiral cleavage describe two different patterns of cell division that occur during cleavage in an embryo. Deuterostomes, organisms such as echinoderms, chordates, arrow worms, and acorn worms, undergo radial cleavage. All other animals with a coelom are called protostomes and undergo spiral cleavage. The cleavage pattern is one of several fundamental embryological differences between the protostomes and deuterostomes.

Cellular mechanisms of development: All multicellular organisms employ the same basic cellular mechanisms of cell movement, induction, determination, pattern formation, expression of homeotic genes, and programmed cell death during their process of development.

Chordates: Three characteristics distinguish the chordates from all other animals: (1) a single, dorsal, hollow nerve cord; (2) a notochord; and (3) pharyngeal slits. These traits appear during the embryological development of the chordate, and in some cases are retained permanently. There are three subphyla of chordates: the lancelets, the tunicates, and the vertebrates.

Amniotic egg: The evolution of the amniotic egg was a major advance for terrestrial vertebrates. It has helped the reptiles and birds become extremely successful terrestrially. Besides protecting the embryo from desiccation, the egg contains a large yolk to nourish the developing embryo. The presence of this yolk influences the cleavage and development patterns that take place.

Amnion and chorion: The amnion and the chorion are two of the internal cell layers or membranes that form in the amniotic egg. The chorion is the outermost layer that lines the inner shell membrane, and the amnion is the innermost layer, forming a sac around the embryo. The amnion and chorion are retained in mammalian embryology; the amnion again encloses the embryo and the chorion helps form part of the placenta.

Terrestrial reproductive strategies: The single biggest problem facing all terrestrial organisms is drying out. Plants were the first organisms to colonize the land about 410 million years ago. Fungi and arthropods soon followed, and ancestral amphibians moved on to the land about 360 million years ago. The truly terrestrial vertebrates – the reptiles, birds, and mammals – have evolved various adaptations or strategies such as internal fertilization, the amniotic egg, and the placenta for solving the problems of reproducing in a terrestrial, desiccating environment. A suite of life history traits helps maximize the organism's reproductive success.

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