564 GLOSSARY 



Dactyl (dfik' til). G.; finger. 



Darwin, Charles R. English naturalist; 1809-1882. 



Darwin, Erasmus. English physiologist; 1731-1802. 



de- (de). L.; down, from, off. 



degeneration (de j6n er a' shtln). The simplification or loss of structure or function 



in an animal organism. 

 delamination (de lam I na' shtin). The splitting off of a layer of cells. 

 dendrite (den' drit). A branched fiber from a nerve cell which carries impulses 



toward the cell body; same as dendron; adj., dendritic (d6n drIt' ik). 

 dent (d6nt). L.; tooth. 

 derm (durm). G.; skin. 

 dermis (dur' mis). The inner connective tissue layer of the vertebrate skin; adj., 



dermal, 

 desiccation (d6s I ka' shiin). Drying up. 

 De Vries (de vres'), Hugo. Dutch botanist; 1848- . 

 di- (dl). G.; two, double. 

 dia- (dl' a). G.; through, apart. 

 dialysis (di ill' I sis). The separation of substances in solution or suspension into 



colloids and crystalloids by means of a permeable membrane. 

 diaphragm (di' a frS,m). The partition between the thoracic and abdominal cavities 



of a mammal. 

 diastase (di' a stas). An enzyme which changes starches to sugars. 

 diecious (di c' shtis). A species of animal in which the male and female organs are 



in different individuals. 

 diencephalon (di en s6f' a I6n). The second region of the vertebrate brain, including 



the optic thalami, optic tracts, and pineal and pituitary bodies; also called 



thalamencephalon. 

 differentiation. The process by which parts differing in structure and function are 



produced from a whole which originally was all alike. 

 digestion (dl jes' chiin). The changing of food to a liquid and absorbable form; 



adj., digestive, 

 dihybrid (di hi' brid). The offspring of parents differing by two characters. 

 dimorphism (di mor' fiz'ni). The appearance of a species of animal in two forms; 



adj., dimorphic, 

 diplo (dip' 16). G.; double. 



diploblastic (dip 16 bias' tik). Composed of two germ layers. 

 diploid (dip'loid). A term applied to the full number of chromosomes found in 



somatic cells. 

 discoidal cleavage. Cleavage in which the blastomeres form a disc at the animal 



pole of the egg cell. 

 dissimilation (dl sIm I la' shiin). The breaking-down processes in a cell by means 



of which chemical compounds are reduced to simpler form and energy is set free; 



adj., dissimilative (dl sIm' I la tlv). 

 divergence (dl vur' j6ns). The acquirement of different characteristics by related 



forms when placed under different environmental conditions, 

 division of labor. The distribution of specific functions to separate parts. 

 dominance (dom' I nilns). The suppression of one character by a contrasted charac- 

 ter when the genes which correspond to the two are both present; also the exercise 



of control by one part of the body over another. 

 Dujardin (dii zhar dftn'), Felix. French zoologist; 1801-1860. 

 duodenum (du 6 de' num). The first part of the small intestine in mammals; adj., 



duodenal (du 6 de' nil). 

 Durant (du rant'), William J. American author; 1885- . 

 dyad. A chromosomal mass formed by the division of a tetrad 



