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one of the embryonic membranes of reptiles, birds, and mammals; encloses a fluid-filled cavity that envelops the embryo

Amniote egg

one of the embryonic membranes of reptiles, birds, and mammals; encloses a fluid-filled cavity that envelops the embryo


a form of connective tissue that forms portions of the skeleton; consists of chondrocytes and their extracellular secretion of collagen; resembles flexible bone


a animal that has a skull

Mammary gland

a milk-producing gland used by female mammals to nourish their young


a mammal whose young are born at an extremely immature stage and undergo further development in a pouch while they remain attached to a mammary gland; includes kangaroos, opossums, and koalas.


a mammal that lays eggs; for example, the platypus.

Nerve Cord

a paired neural structure in most animals that conducts nervous signals to and from the ganglia; in chordates, a nervous structure lying along the dorsal side of the body; also called spinal cord. .


a stiff but somewhat flexible, supportive rod found in all members of the phylum Chordata at some stage of development.

pharyngeal gill slit

an opening, located just posterior to the mouth, that connects the digestive tube to the outside environment; present (as some stage of life) in all chordates.


complex interweaving of the uterine lining and the embryonic membranes, especially the chorion; functions in gas, nutrient, and waste exchange between embryonic and maternal circulatory systems and secretes hormones.


referring to a mammal, possessing a placenta (that is, species that are not marsupials or monotremes).

post-anal tail

a tail that extends beyond the anus; exhibited by all chordates at some stage of development.

vertebral column

a column of serially arranged skeletal units (the vertebrae) that enclose the nerve cord in vertebrates; the backbone.


an animal that possesses a vertebral column.

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