Zooxanthellae. Unicellular algae, living within cells of a variety of aquatic animals of their mutual advantage; especially important in the coelentrates of coral reefs.

Zygomatic Arch. In man and other mammals, bony arch forming border of orbit towards back of head, to which main chewing muscle (masseter) is attached.

Zygomorphic. Exhibiting bilateral symmetry

Zygote. A fertilized female gamete; the product of the fusion of the nucleus of the ovum or ovule with the nucleus of the sperm or pollen grain.

Zygotene. Stage in prophase of first division of meiosis following leptotene, in which pairing (synapsis) of homologous chromosome occur with formation of bivalents.

Zymase. Enzyme system of yeast which breaks down (ferments) hexose sugars into alcohol and carbon dioxide.

Zymogen. Inactive form of an enzyme, secreted by a cell, which is converted to activity usually by excision of a polypeptide.

Zymogen Granule. A Cytoplasmic particle surrounded by a membrane formed by the Golgi apparatus. It stores and secretes zymogen.