The embryo has three germ layers. The ectoderm, mesoderm, and endoderm. The endoderm is the innermost layer. Other than the endocrine glands, the other tissues that develop from the endoderm are the gastrointestinal tract, urinary and auditory system, and the respiratory tract.
Ionic bonds result when electrons are tranfered between atoms. Covalent bonds result when valence electrons are shared between atoms. You should be able to identify whether an ionic or covalent bond is being formed between atoms.
