Appendix 819 



Bilateral symmetry, arrangement of parts on opposite sides of a certain plane 



so that they are similar to each other. 

 Binary fission (bi' na ri; fish' un) (L. bini, two by two; fissura, to split), division 



into two equal parts. 

 Binomial nomenclature, the scientific method of naming organisms by two Latin 



or latinized words, the first the genus, the second the species. 

 Biochemistry, the chemical aspects of biologic phenomena. 

 Biogenesis (bi o -jen' e sis) (Gr. bios, life; genesis, origin), the law that all life 



arises from preexisting living matter (contrast with Abiogenesis). 

 Biogenetic theory, each individual in its embryologic development (ontogeny) 



repeats or recapitulates, in modified or abbreviated form, the stages in the 



evolutionary development of that race (phylogeny). In other words, 



ontogeny repeats phylogeny. 



Phylogenetic Stages Ontogenetic Stages 



(1) Single-celled animal (1) Egg (single cell ) 



(2) Solid mass of cells (2) Morula and blastula stage 



(3) Two-layered animal (3) Gastrula (two layers) 



(4) Three-layered animal (4) Three-layered embryo 



Biogeography (bi o je -og' ra fi) (Gr. bios, life; geo, earth; graphein, to write), 

 geographic distributions of organisms in space {see Zoogeography and Phy- 

 togeography). 



Biology (biol'o ji) (Gr. bios, life; logos, science), study of living things. 



Bioluminescence (bi o lu mi -nes' ens) (L. luminis, light), light emission by living 

 organisms not directly attributable to heat that produces incandescence. 



Bionomics (bi o -nom' iks) (Gr. bios, life; nomos, law), the relations of living 

 organisms to their environments {see Ecology). 



Biophysics (bi o -fiz' iks) (Gr. bios, life; L. physica, natural), the physical aspects 

 of biologic phenomena. 



Biparental (bi pa -rent' al) (Gr. bi, two), having a male and a female parent. 



Biramous (bi-ra'mus) (Gr. bi, two; ramus, branch), having two branches. 



Bisexual (bi -sex' shual) (Gr. bi, two; sexus, sex), possessing both male and female 

 sex organs. 



Bivalent chromosomes (bi -va' lent; kro' mo som) (Gr. bi, two; valere, to have 

 power), two chromosomes, one from the male and the other from the fe- 

 male, united temporarily. 



Bivium (biv' i um) (Gr. bi, two; via, way), one side of an echinoderm having two 

 rays. 



Bladder worm (A.S. blaedere, bag), baglike stage of embryonic tapeworm. 



Blastocoel (blas'tosel) (Gr. blastos, bud; koilos, hollow), hollow segmentation 

 cavity of the embryo. 



Blastocyst (blastodermic vesicle) (bias' to sist) (Gr. blastos, bud or young; kystis, 

 sac), the hollow stage which follows the embryonic morula stage. 



Blastoderm (bias' to durm) (Gr. blastos, young; derma, skin), a cellular mem- 

 brane formed by the division of the blastomeres. 



Blastomere (bias' to mer) (Gr. blastos, young; meros, part), any cell formed by 

 mitosis of the egg. 



Blastopore (blas'topor) (Gr. blastos, young; poros, pore), the pore of the blas- 

 tula stage. 



