X.
Xi´phoid, a. or n. Sword-shaped. Posterior prolongation of the sternum.
Y.
Yolk or Yelk, n. “Yellow” of an egg; a soft, opaque, yellow globular mass, enveloped in a proper membrane (the vitelline) and suspended in the white by the chalazæ. It has a central cavity full of a clear substance, furnished with a duct leading to a mass of cells termed proligerous disc; adhering to its surface is the cicatriculum, or “tread,” which becomes the embryo.
Z.
Zo´na Pellu´cida, n. Consolidated membranous envelope of the primitive ovum.
Zoölogy, n. Natural history of animals, of which ornithology is one department.
Zygapoph´ysis, a. Articular process of a vertebra.
Zygodac´tyle, a. Yoke-toed; having the toes in pairs, two before and two behind.
Zygo´ma, n. Malar bone and its connections.