GLOSSARY OF EMBRYOLOGICAL TERMS 293 



Omnipotent — term used in connection with a cell which could, under 

 various conditions, assume every cytological differentiation known to 

 the species or which, by division, could give rise to such varied differ- 

 entiations. 



"Omnis Cellula e Cellula" — all cells from preexisting cells (Virchow). 



Ontogeny — developmental history of an organism; sequence of stages in 

 early development. 



Oocyte — presumptive egg cell after initiation of growth phase of maturation. 

 (Syn., ovocyte.) 



Oogenesis— process of maturation of ovum; transformation of oogonium 

 to mature ovum. (Syn., ovogenesis.) 



Oogonia — multiplication (mitotic) stage prior to maturation of presumptive 

 egg cell (ovum), found most frequently in peripheral germinal epithe- 

 lium. 



Ooplasm — cytoplasmic substances connected with building rather than re- 

 serve materials utilized in developmental processes. 



Opercular Chamber — See Branchial Chamber. 



Operculum — integumentary growth posteriorly from each of the hyoid 

 arches of frog embryo, which covers and encloses gills. 



Optic Chiasma — thickening in forebrain ventral to infundibulum, found 

 as a bunch of optic nerve fibers in future diencephalon. 



Optic Cup — invagination of outer wall of primary optic vesicle to form 

 a secondary optic vesicle made up of two layers; a thick internal or 

 retinal layer continuous at pupil and choroid fissure, and a thin external 

 layer which is pigmented. Cavity of cup becomes future posterior 

 chamber of eye. 



Optic Lobes — thickened, evaginated, dorso-lateral walls of mesencephalon. 



Optic Recess — depression in forebrain anterior to optic chiasma which 

 leads to optic stalks. 



Optic Stalk — attachment of optic vesicle to forebrain, at first a tubular 

 connection between optic vesicle and diencephalon. Lumen is later 

 obliterated by development of optic nerve fibers. 



Optic Vesicle — evagination of forebrain ectoderm to form primary optic 

 vesicles which in turn invaginate to form secondary optic vesicles or 

 optic cups of eyes. 



Opticoel — cavity of primary optic cup. 



