GLOSSARY 445 



Dentine. A bony structure just under the enamel and forming the bulk 

 of the mammalian tooth and of the placoid scale. 



Dermal bones. Bones derived directly from mesoderm without passing 

 through a cartilage stage; phylogenetically derived from dermal 

 scales. 



Dermis. The inner layer of the integument of vertebrates. 



Diaphragm. A muscular partition that in mammals separates the peri- 

 toneal cavity from the pleural cavities and is employed in the process 

 of breathing. 



Diastase. A starch-splitting enzyme. 



Differentiation. The term is employed to include the processes 

 whereby cells become structurally and functionally different as they 

 assume special functions in the organism. 



Digestion. The conversion of foods into physical and chemical states 

 in which they are available for use in metabolism. 



Di-HYBRiD. The progeny of a cross between two parents that differ in 

 two pairs of alternative characters. 



DiPLOBLASTic (Adjective). An animal that consists of two layers of cells 

 and their derivatives. 



Diploid number. The number of chromosomes characteristic of the 

 nuclei of the tissue cells. 



Dominant. A member of a pair of alternative characters is said to be 

 dominant when the progeny of a cross between a parent having 

 the character and one having its alternative all resemble the 

 first-mentioned parent with respect to that particular character. 



Ecology. The science which deals with the relation between the or- 

 ganism and its environment. 



Ectoderm. The outer layer of cells in the early stages in the develop- 

 ment of the Metazoa. 



Ectoplasm. The protoplasm which forms the ectosarc. 



EcTosARC. Living protoplasm forming the region just under the boundary 

 (plasma membrane) of the cell; applied particularly to the Protozoa. 



Effector. An organ that responds to an impulse. The chief effectors 

 are muscles and glands. 



Electrolyte. A substance that dissociates into ions when dissolved. 



Electron. A component of an atom consisting of a negative electrical 

 charge. 



Element. A substance that consists of atoms all of the same sort. 



Embryo. A new individual in the process of development. 



