GLOSSARY 467 



Dynamic. Pertaining to change or process. Characterized by energy or 

 action. 



Ecology. The study of the relation of organisms to their environment, 

 both animate and inanimate. 



Ectoderm. The outer layer of the gastrula. 



Ectoparasite. External parasite. 



Egg. The female germ cell, either before or after fertilization. 



Embryo. An animal in the early stages of its development before it is 

 liberated from the egg membranes. 



Embryogeny. The formation of the embryo and the course of its develop- 

 ment. Embryogenesis. Embryology is the study of embryogeny. 



Encystment. The formation of a protective covering about an organism, 

 particularly Protozoa. 



Endocrine gland. See Ductless gland. 



Endoderm. The germ layer forming the lining of the gastrula and bound- 

 ing the archenteron or gastrocoel; lining of the alimentary canal. 



Endomixis. A nuclear reorganization occurring in Protozoa without 

 conjugation and therefore without synkaryon formation (fertilization). 



Endoparasite. Internal parasite. 



Endoskeleton. An internal living skeleton such as is present in 

 vertebrates. 



Entelechy. A word adopted from Aristotle by Hans Driesch to desig- 

 nate the "vital force," which he believes is necessary as an addi- 

 tional factor outside the range of known forms of energy to explain life. 

 Entelechy and the "soul" of Descartes are practically the same con- 

 cept, except that Descartes entertained the fanciful notion that the 

 soul resided in the pineal gland, which is a dorsal outgrowth of the roof 

 of the forebrain and in all probability represents an ancestral eye. 



Enzyme. An organic catalytic agent which accelerates chemical reaction 

 in the body. 



Epigenesis. The idea that an organism in its development starts as a 

 relatively homogeneous initial plasma which becomes heterogeneous 

 as a result of the action of external factors upon it. The opposite of 

 preformation in development. 



Epithelium. A layer of cells covering an external or internal surface of the 

 body. 



Eugenics. The science of improving the inborn qualities of the human 

 race by better breeding. 



Eustachian tube. A duct connecting the middle-ear cavity with the 

 pharynx. It is a survival of the first gill cleft of the fish. 



Eutheria. A subclass including the viviparous mammals. 



Evagination. The outgrowth of a pocket of cells from a surface. 



Evolution. The doctrine that organisms of today are derived by descent 

 from those of the past; that organisms have changed from time to time; 

 that, in general, higher organisms are descended from lower ones. 



Excretory. Pertaining to waste substances formed in metabolism. 



Exoskeleton. The lifeless external cuticle forming the protective covering 

 and supporting framework of arthropods. An external skeleton. 



