GLOSSARY 435 



DENITRIFYING BACTERIA. Types of Bacteria which break down 

 compounds of nitrogen and set free the nitrogen. 



DERMAL. Pertaining to the skin. The dermis is the inner layer of 

 the Vertebrate skin. See Epidermis. 



DIFFERENTIATION. A transformation from relative homogeneity to 

 heterogeneity, involving the production of specific substances or 

 parts from a general substance or part. Specialization. 



DIHYBRID. The progeny of parents differing in regard to two given 

 characters. 



DIPLOID. The maximum or full (duplex) number of chromosomes 

 which occurs during 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 

 which appears to the exclusion of the other (recessive) character. 



DORSAL AORTA. Chief artery distributing pure blood to the body. 

 Ventral aorta carries blood from heart to gill-arteries in Fishes. 



DUCTLESS GLAND. An organ whose function is to elaborate and se- 

 crete a hormone directly into the blood. An endocrine gland. 



ECOLOGY. The study of the relations of the organism to environing 

 conditions, organic and inorganic. 



ECTODERM. The primary tissue comprising the surface layer of cells 

 in the gastrula; its derivatives in subsequent stages forming the 

 outer part of the skin, nervous system, etc. See Germ Layer. 



ECTOPLASM. Modified surface layer of cytoplasm of a cell. See 

 Endoplasm. 



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. 



EGG. The female gamete. Ovum. 



EMBRYOLOGY. The study of the early development of individual 

 organisms. 



EMBRYO SAC. Megaspore of the Flowering Plants. 



EMULSOID. A state in which one liquid is divided into very fine drop- 

 lets and suspended in another liquid with which it is immiscible. 



