436 APPENDIX 



ENCYSTMENT. The formation of a resistant covering, or cyst wall, 



about an organism. 



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., 



Paramecium, which does not involve the cooperation of two 



cells (as in conjugation) and therefore is without synkaryon 



formation. 

 ENDOPLASM. The inner cytoplasm surrounding the nucleus; e.g., 



in Paramecium. See Ectoplasm. 

 ENDOPODITE. The inner of the two distal parts of the typical bira- 



mous Crustacean appendage. See Protopodite and Exopodite. 

 ENDOSKELETON. An internal living skeleton affording support 



and protection, as well as levers for the attachment of muscles. 



Characteristic of Vertebrates. 

 ENDOSPERM. A tissue, containing reserve food materials, formed 



within the embryo sac. 

 ENTERIC CAVITY. The digestive cavity of the gastrula stage, and of 



simple Metazoa, e.g., Hydra. 

 ENZYMES. Complex chemical substances of organisms which bring 



about by catalytic action many of the chemical processes of the 



body; e.g., digestion. 



EPIDERMIS. The outer cellular layer of the skin. 

 EPIGENESIS. Development from absolute or relative simplicity to 



complexity. See Preformation. 

 EPITHELIUM. A layer of cells covering an external or internal suf- 



face, including the essential secreting cells of glands. 

 EQUATION DIVISION. A typical division of the nucleus involving 



division of the chromosomes. See Reduction Division. 

 EQUATORIAL PLATE. The equator of the spindle with its group of 



chromosomes during the metaphase of mitosis. 

 EUGENICS. The system of improving the human race by breeding 



the best. "The science of being well born." See Euthenics. 



