Mammal

• chorion

  The outermost membranous sac enclosing the embryo in higher vertebrates (reptiles, birds...

• amnion

  Thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles,...

Also contains

• pocket

  (anatomy) saclike structure in any of various animals (as a marsupial or gopher or pelic...

• vesicle

  A small anatomically normal sac or bladderlike structure (especially one containing flu...

• bladder

  A distensible membranous sac (usually containing liquid or gas).

• air sac

  A tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air pa...

• saccule

  A small sac or pouch (especially the smaller chamber of the membranous labyrinth).

• air bladder

  An air-filled sac near the spinal column in many fishes that helps maintain buoyanc...

• coelenteron

  The saclike body cavity of a coelenterate.

• air sac

  Any of the membranous air-filled extensions of the lungs of birds.

• air sac

  Any of the thin-walled extensions of the tracheae of insects.

• yolk sac

  Membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some...

• yolk sac

  Membranous structure that functions as the circulatory system in mammalian embryos until...

• acinus

  One of the small sacs or saclike dilations in a compound gland.

• bursa

  A small fluid-filled sac located between movable parts of the body especially at joints.

• pericardial sac

  The membrane surrounding the heart.

• cistern

  A sac or cavity containing fluid especially lymph or cerebrospinal fluid.