vesicle (ves'ik-1). 1

A little sac or cyst; in histology, a minute solid or semisolid ma (e. g., a cell or nucleus) inclosed by a membrane. 2. A lesion of the skin or mucous membrane formed by a collection of clear liquid within or beneath the epithelium, causing a small, circumscribed elevation of the surface. See bleb, bulla, and pock, allantoic v. The internal, epithelial portion of the allantois. anterior cephalic. A thin walled cavity formed by the closing in of the dorsal laminae over the cephalic extremity of the primitive groove of the embryo and a subsequent transverse constriction at about the extremity of the notochord. From it are developed the cerebral hemispheres, pineal gland, pituitary body, corpus callosum, cerebral peduncles, fornix, and lateral and third ventricles. Subsequently it is divided by an anteroposterior cleft, which finally becomes the longitudinal fi ure, blastodermic v. See blastula. encephalic v's. The three compartments into which the anterior extremity of the primitive medullary tube is divided at an early period in fetal life. Other secondary v's spring from the primary one, from which are formed the organs of sense, fat v. See fat cell, under cell. germinal v., germinative v. A somewhat spherical body in the ovum with an investing membrane, having fluid and granular contents with a protoplasmic network and germinal spot or spots; regarded e entially as a nucleus, graafian v. Syn.: follicle of de Graaf, ovarian follicle. A closed sac which contains the ovarian ovum. It consists of an external membrane lined with small granular cells (the membrana granulosa) which are condensed at one point to form the discus pro liger us. Prior to the ripening of the ovum the v. is situated within the stroma of the ovary, but as maturation advances it becomes superficial, its walls grow vascular, and it is distended with a serous fluid. It finally ruptures, allowing the ovum to escape, middle cerebral y, VESSEL In embryology, the dilatation of the medullary tube just posterior to the cephalic extremity of the notochord, from which the corpora quadrigemina, cerebral peduncles, and aqueduct of Sylvius are developed, optic v. See primary ocular v. otic v. Syn.: auditory capsule. The embryonic rudiment of the organ of hearing, formed by an involution of the epiblast from the outer surface of the head in the region of the medulla oblong at a, opposite the dorsal end of the hyoidean arch, primary ocular (or optic) v's. Lateral projections from the anterior cerebral v., from which the eyes are developed. primitive cerebral v. An expansion at the anterior extremity of the medullary tube, from which is developed the encephalon. It is subsequently divided into the anterior, middle, and posterior cerebral v's. seminal v's. Two membranous tubes, coiled and doubled upon themselves so as to present a sacculated appearance, lying on the posterior surface of the base of the bladder with their blind posterior extremities (between which the vasa deferent i a run) considerably separated. They converge toward the prostate and unite each with its corresponding vas deferens. They serve as receptacles for the semen, umbilical v. The navel sac, a structure of the human ovum which in the lower animals is called the vitelline sac or yolk sac. It is situated on the abdominal aspect of the embryo, being continuous with the omphalomesenteric duct. [Lat., vesicula, dim. of vesica, a bladder.]