GLOSSARY 



DISCHARGE. The passage of a nerve impulse from a nerve fiber to an eflfector. 

 DISJUNCTION. The separation of the chromosomes of a homologous pair and their 



distribution into separate cells during one of the meiotic divisions. CJ. 



SEGREGATION. 

 DISTAL. Situated away from the center or place of attachment. CJ. PROXIMAL. 

 DIURNAL. Pertaining to the davtime; daily. CJ. NOCTURNAL. 

 DOMESTICATION. The taming or rearing of wild animals by man, with the consequent 



changes in their environments and habits of life. 

 DOMINANCE. The differentiation of a region under the influence of one gene, called 



the dominant gene, although an allelomorph, called the recessive gene, may be 



present; where dominance occurs, a heterozygous individual, with a single 



dominant gene, looks like a homozygous individual with two dominant genes 



but does not breed true. 

 DORSAL. Pertaining to or situated near the back or upper surface of an animal or 



part. Cf. VENTRAL. 

 DUCT. A tube, or canal, by which a liquid is conducted; usually restricted to such 



a tube that opens on a .surface. Cf. VESSEL. 

 DUCTLESS GLAND. See ENDOCRINE GLAND. 

 DUCTUS {pi. DUCTUS). See DUCT. 



ECOLOGICAL SUCCESSION. The orderly temporal sequence of forms succeeding each 



other in an environment undergoing gradual changes in its general conditions. 



As each environmental condition changes, forms previously adapted to existence 



in the environment either die out or emigrate, to be replaced by other organisms 



adapted to the new conditions. 

 ECOLOGY. The science of the relations of organisms to their external environment. 

 ECTODERM. The outermost of the two germ layers formed during gastrulation; covers 



the gastrula. Cf. ENDODERM and MESODERM. 

 ECTOPARASITE. A parasite that lives on the outside of its host. CJ. ENDOPARASITE. 

 ECTOPLASM. The outermost layer of the cell contents in Protozoa. 

 ECTOTHERMOUS. Animals having no capacity, or at most a very limited capacity, 



of maintaining their own body temperatures constant in the face of changes in 



the environmental temperature; loosely, "cold-blooded." 

 EFFECTOR. A region where a reaction occurs as a result of a stimulus; muscles and 



glands are effectors. 

 EFFERENT. Carrying away from a given region. CJ. AFFERENT. 

 EGESTION. The elimination of indigestible or undigested material from a place of 



digestion. CJ. INGESTION. 

 EGG AXIS. An imaginary line passing through the center of the egg and the point 



where the first polar body is formed; connects the animal pole of the egg with 



the vegetal pole. 

 EMBRYO. A young organism before its hatching or birth and before it is self- 

 sustaining. CJ. LARVA and JUVENILE. 

 EMBRYOLOGY. The science of development. 

 EMBRYONIC CELL. See TOTIPOTENT CELL. 

 EMBRYONIC MEMBRANES. Cellular membranes formed in association with the embryo 



during development and related to its metabolic requirements; specifically, the 



yolk sac, amnion, chorion, and allantois in vertebrates. 

 ENCYST. To become enclosed in a cyst. 



ENCYSTMENT. Process of encasement within a cyst. C/. EXCYSTMENT. 

 ENDOCRINE. A secretion produced by an endocrine or ductless gland and serving 



as a stimulus in chemical coordination; sometimes called a hormone. 

 ENDOCRINE GLAND. A gland without a duct, the secretion of which passes into the 



blood stream. 



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