GLOSSARY-INDEX 411 



omosternum (Gr. omos = shoulder + sternon = the chest). A bit of 

 cartilage forming the front of the sternum in the frog, 182. 



ontogeny (Gr. ont = being + -geneia = a producing). Development from 

 the egg, 19, 290. 



oocyte (Gr. oon = egg + cytos). An egg before maturation, 252. 



oogenesis (Gr. oon = an egg + genesis = creation). The development of 

 the egg in the ovary, 252. 



oogonium (Gr. oon = an egg + gonos = offspring). A sac in some plants 

 within which are produced one or two eggs. 



operculum. A lid-like cover. 



optic lobes. The section of the brain in front of the cerebellum; the mid- 

 brain, 193. 



optic nerve. The second cerebral nerve, supplying the eye, 194, 196. 



optimum temperature, 132. 



oral. Pertaining to the mouth, 60. 



order. A group of similar families, 372. 



organ. Any part of an animal or plant adapted for a specific function; 

 usually made of a combination of several different tissues, 26, 95. 



organic evolution, 349. 



organic substances. Substances originally derived from organisms, 26; 

 in chemistry, any compounds of carbon. 



organism. A living being provided with organs; hence any living being, 

 26. 



organisms as machines, 298. 



origin. The attachment of a muscle nearest the center of the body, 184. 



Oscittaria, 136, 219. 



osmosis. The force that causes some substances to diffuse through mem- 

 branes which are moistened on both sides; dialysis, 127, 307. 



ossification. Turning to bone. 



osteoblast (Gr. osteon = a bone + blastos = a sprout). A bone-forming 

 cell. 



otic. Pertaining to the ear, 180. 



otocyst (Gr. ous = the ear + cystis = a sac). A sac which in many 

 invertebrates is supposed to have hearing functions. 



otoliths (Gr. ous = the ear -f lithos = a stone). Calcareous bodies found 

 in the otocysts in some animals. 



outgo, of an animal, 220; of a plant, 220. 



ova. The female reproductive cells, 249, 267. 



ovary. In animals the glands producing eggs, 151, 163, 200, 249; in flowers 

 the lower part of the pistil containing the ovules and seeds, 120. 



overproduction, 353. 



oviducts. Ducts for carrying eggs to the exterior, 163, 200, 249. 



