276 GLOSSARY OF EMBRYOLOGICAL TERMS 



Entelchy — Driesch's theory of an (intangible) agent controlling develop- 

 ment. (Syn., elan vital.) 



Enterocoel — cavity or pouch within mesoderm just formed by evagination 

 of gut (enteron) endoderm as in Amphioxus. (Syn., gut pouch, 

 coelomic pouch, archenteric pouch.) 



Enteron — definitive gut of embryo, always lined with endoderm. 



Ento-mesoderm — refers to portion of invaginating blastoporal lips which 

 will induce formation of medullary fields in amphibian embryo. 



Entrance Cone — temporary depression on surface of egg following en- 

 trance of spermatozoon. 



Entrance Path — See Path, penetration. 



Ependymal Cells — narrow zone of non-nervous and ciliated cells which 

 surround central canal (neurocoel), from outer ends of which 

 branching processes extend to periphery, such processes forming a 

 framework for other cellular elements in spinal cord and brain. 



Epiblast — outermost layer of early embryo from which the various germ 

 layers may be derived. 



Epiboly — growing, spreading, or flowing over; process by which rapidly 

 dividing animal pole cells or micromeres grow over and enclose 

 vegetal pole material. Increase in areal extent of ectoderm. 



Epibranchial Placode — placode (thickening) external to gills related to 

 lateral line organs and tenth cranial nerves, (Syn., suprabranchial 

 placode.) 



Epidermis — ectodermal portion of skin including cutaneous glands, hair, 

 feathers, nails, hoofs, and some types of horns and scales. 



Epigenesis — development of systems starting with primitive, homogene- 

 ous, lowly organized condition and achieving great diversification. 



Epimere — most dorsal mesoderm, that lying on either side of nerve and 

 notochord, which gives rise to somites. (Syn., axial mesoderm.) 



Epiphysis — evagination of anterior diencephalon of vertebrates which be- 

 comes separated from brain as pineal (endocrine) gland of adult. 



Epithelioid Bodies — endodermal masses arising from second and third 

 visceral pouches of amphibia. 



Epithelium — thin covering layer of cells; may be ectodermal, endodermal, 

 or mesodermal. 



