Hypophysis.
(1) (Hypophyseal Pouch) Median ectodermal inpushing of embryonic vertebrate head, just in front of buccal membrance, which fuses with infundibulum. Forming pituitary gland.
(2) Synonymous with pituitary body.

Hupopnea. The decreased ventilation of the lungs.

Hypotasis. Interaction between non-allelic genes in which one (hypostatic) gene will be expressed in presence of a second.

Hypostome. Any structure around or below the mouth such as the oral one in hydra.

Hypothalamous.
Floor and sides of brain of vertebrates just behind attachment of cerebral hemispheres. Derived from fore-brain. In mammals it is to contain centres co-ordinating, amongst other things, manifestation of rage and mechanism of body temperature control, both of which predominantly involve sympathetic nervous system.

Hypotonic.  (Of a sodium) having a concentration so that it loses water by osmosis across a semi-permeable membrane to some specified other solution. when the membrane and the other solution are unspecified they are taken to be the plasma membranes and interior of cells respectively.

Hypoxia. The reduction of oxygen levels.

Hzone. The lightest area in the middle of a muscle sarcomere myosin and actin filaments do not overlap.