294 GLOSSARY OF EMBRYOLOGICAL TERMS 



Oral Plate — stomodeal ectoderm and pharyngeal endoderm fused to form 

 oral membrane. Breaks through to form mouth. (Syn., pharyngeal 

 membrane, oral membrane, stomodeal plate.) 



Oral Suckers — elongated, pigmented depressions at antero-ventral ends 

 of mandibular arches of frog embryo which give rise to mucous glands; 

 with adhesive function. 



Organization — indicated by interdependence of parts and the whole. "When 

 elements of a certain degree of complexity become organized into an 

 entity belonging to a higher level of organization," says Waddington, 

 "we must suppose that the coherence of the higher level depends on 

 properties which the isolated elements indeed possessed but which 

 could not be entered into certain relations with one another." See 

 Gestalten. 



Organizer — chorda-mesodermal field of amphibian embryo; a tissue area 

 which has power of organizing indifferent tissue into a neural axis; 

 possibly comparable to Henson's node of chick embryo. 



Osteoblasts — mesenchymal cells which actively secrete a calcareous ma- 

 terial in formation of bone; bone-forming cells. 



Osteoclasts — bone-destroying cells; cells which appear in and tend to 

 destroy formed bone; constantly active, even in embryo. 



Ostium Abdominale Tubae — most anterior, fimbriated end of oviduct in 

 female vertebrates; point of entrance of ovulated egg into oviduct; 

 double in amphibia. (Syn., infundibulum of oviduct, tubal ridge.) 



Otic Vesicle — auditory vesicle, otocyst. 



Otocyst — original auditory vesicle appearing at level of rhombencephalon 

 in amphibian embryo just before hatching, forming first as a placode. 

 (Syn., auditory vesicle.) 



Oviducal Membranes of Ovum — tertiary membranes applied over egg as 

 it passes through oviduct. 



Oviducts — paired MUllerian ducts in both males and females, which gen- 

 erally persist in males. 



Ovigerous Cords — columns or strands of tissue which divide germinal 

 epithelium of primordium of ovary, carrying primordial germ cells 

 with them and later breaking up into nests of cells, each of which 

 contains an oogonium. (Syn., egg tubes or cords of Pfliiger [mammal].) 



Oviposition — process of laying eggs. 



Ovocyte — See Oocyte. 



