GLOSSARY 497 



Diffusion. Intermingling of two substances due to migration of their 

 molecules. Pressure exerted by molecules in diffusion through a semi- 

 permeable membrane is osmotic pressure. See Osmosis. 



Digestion. Chemical simplification of food so that it can be absorbed 

 and utilized. 



Dihybrid. Progeny of parents differing in two given characters. 



Diploblastic. Composed of only two primary layers: ectoderm and endo- 

 derm, e.g., Hydra. See Triploblastic. 



Diploid. Having two complete sets of homologous chromosomes. Max- 

 imum number of chromosomes in the life history of a given species. See 

 Haploid. 



Division of Labor. Allocation of special functions to special parts which 

 cooperate toward the unity of the whole. 



Dominant Character. One of a pair of alternative characters, repre- 

 sented by homologous genes, which appears in the phenotype to the 

 exclusion of the other (recessive) character when both are present in 

 the genotype. 



Dorsal Aorta. Chief artery distributing pure blood to the body. Ven- 

 tral aorta carries blood from heart to gill arteries in Fishes. 



Ductless Gland. An organ whose function is to elaborate and secrete a 

 hormone directly into the blood. An endocrine gland. 



Ecology. The study of the relations of the organism to environing condi- 

 tions, organic and inorganic. 



Ectoderm. The primary tissue comprising the surface layer of cells in the 

 gastrula. See Germ Layer. 



Ectoplasm. Modified surface layer of cytoplasm of a cell. See En- 

 doplasm. 



Efferent Root. Ventral, or anterior, root of certain cranial and all spinal 

 nerves through which motor nerve impulses leave the brain and spinal 

 cord. See Afferent Root. 



Embryology. Study of the early developmental stages, or embryos, of 

 individual organisms. 



Encystment. The formation of a resistant covering, or cyst wall, about 

 an organism; e.g., Euglena. 



Endocrine Gland. See Ductless Gland. 



Endoderm. The primary tissue comprising the inner layer of cells 

 in the gastrula, and in subsequent stages forming the lining of the 

 essential parts of the digestive tract and its derivatives. See Germ 

 Layer. 



Endomixis. A nuclear reorganization process in Protozoa, e.g., Parame- 

 cium, which does not involve the cooperation of two cells (as in conjuga- 

 tion) or synkaryon formation. 



