Glossary - 687 



teristic component of the nucleus in which DNA is 

 always represented. 



Chromidia. Granules of chromatin scattered through 

 the cell, chiefly in cells that have no distinct nu- 

 cleus and cytoplasm. 



Chromosomes. Threads or rods of chromatin (DNA- 

 protein complex) , which appear during mitosis 

 (and meiosis) . 



Cilia. Fine protoplasmic processes that beat in reg- 

 ular fashion to move the cell or move particles over 

 its surface. 



Class. A taxonomic group, next below a phylum. 



Cleavage. The successive cell divisions of the zygote 

 (or parthenogenetic egg) that transform it into a 

 multicellular embryo. 



Cloaca. The common posterior opening of the en- 

 teron, ureters, and gonoducts in most vertebrates 

 (except mammals) . 



Coarse. Of microscopic or macroscopic size; i.e., 

 above about 100 m^ in diameter. 



Coflom. A mesodermal cavity occupying most of the 

 space between the body wall and the enteron in 

 annelids, vertebrates, etc. 



Cohesion. Attraction between molecules of the same 

 substance. 



Colloidal. Of ultramicroscopic size; i.e., between 

 about 1 mfi and 100 m/x in diameter. Dispersions 

 containing molecules or particles of this size. 



Colonial organism. An aggregate of cells all more 

 or less alike in structure and function. 



Components. The substances making up a mixture. 

 (cf. Constituents.) 



Compound. A substance containing only one kind of 

 molecule, each molecule consisting of two or more 

 kinds of atoms. 



Conditioned reflex. A habitual response to a par- 

 ticular stimulus determined by the previous experi- 

 ence of the individual; i.e., by the frequent associa- 

 tion of this stimulus with a stimulus that originally 

 aroused this response. 



Conductor. A structure specialized for the transmis- 

 sion of excitation. 



Coenzyme. A. Important cofactor that transmits 

 activated acetyl units to the Krebs cycle, in cells 

 generally, (cf. Fig. 8-5.) 



Cofactor. A nonprotein component of many en- 

 zymes; either firmly bonded to the protein, in 

 which case it is called a prosthetic group, or loosely 

 bonded, in which case it is called a coenzyme. 



Conjugation. The union of two gametes; usually 

 used only in cases where the two gametes are 

 alike. 



Connective tissues. Tissues in which the cells are 

 irregularly distributed through a relatively large 

 amount of intercellular material. 



Constituents. The elements making up a com- 

 pound, (cf. Components.) 



Coordination. The production of harmonious inter- 

 action of the various parts and processes of an or- 

 ganism. 



Copulation. The introduction of sperm cells into 

 the body of the female by a male animal. 



Corolla. The whorl of parts (petals) just inside the 

 calyx of a complete flower. 



Cortex. The portion of a stem or root between the 

 epidermis and the fibrovascular bundles. 



Cotyledon, Storage leaf of an embryo sporophyte or 

 seedling. 



Cranial nerves. Nerves arising from the brain. 



Cross-fertilization. Union of gametes from differ- 

 ent individuals, (cf. Self-fertilization.) 



Cross over. The exchange of corresponding portions 

 of homologous chromosomes during synapsis. 



Crystalloidal. Of amicroscopic size; i.e., below 

 about 1 m,u in diameter. Solutions containing mole- 

 cules of this size. 



Cytochromes. A series of iron compounds present 

 in cells generally that relays electrons from level to 

 level, generating useful energy. 



Cytoplasm. The protoplasm of a cell exclusive of 

 the nucleus. 



Death. Irreversible cessation of the activities and 

 breakdown of the structure of protoplasm. 



Dehydration synthesis. The chemical combination 

 of two or more molecules (of the same substance or 

 different substances) with elimination of water. 



Dehydrogenase. Any enzyme that catalyzes the trans- 

 fer of one or more electrons (and hydrogen) from 

 one substance to another. 



Dendrons. Relatively short, usually much branched 

 processes of a neuron, carrying impulses toward the 

 cell body. 



Denitrifying bacteria. Bacteria that decompose 

 nitrogen compounds and liberate free nitrogen. 



Development. The progressive production of the 

 phenotypic characteristics of an organism. 



Differentiation. The structural and functional spe- 

 cialization of different parts. 



Diffusion. The spreading of a dissolved substance 

 through the solvent by virtue of the random move- 

 ments of its molecules or ions. 



Digestion. The hydrolysis of colloidal, sometimes 

 insoluble, food substances into soluble crystalloidal 

 substances. 



Dioecious. Having separate sexes; i.e., the two kinds 

 of gametes produced by different individuals, (cf. 

 Monoecious.) 



Diploid. The number of chromosomes in the zygote, 

 which is twice the number in each gamete, (cf. 

 Haploid.) 



Disaccharide. A carbohydrate that can be hydro- 

 lyzed into two monosaccharides; i.e., each molecule 

 of the disaccharide yields two monosaccharide mole- 

 cules (either alike or different) . 



Dispersion. A mixture in which one or more sub- 

 stances are distributed (dispersed) throughout an- 

 other substance (the dispersion medium) . 



DNA. Deoxyribonucleic acid, in which the sugar of 

 the sugar-phosphate chain is deoxyribose; carrier of 

 the genetic code in cells generally; capable of se-lf- 

 templated synthesis as well as of determining RNA 

 synthesis; closely identified with the genie material. 



Dominant. A gene whose phenotypic effect largely 

 or entirely obscures that of its allelic gene. (cf. 

 Recessive.) 



Dorsal. Pertaining to the upper side (the back in 

 man) . 



