438 CHORDATE ANATOMY 



decussation. A decussation of nerve fibers occurs when they cross the median plane to 



connect unlike centers on the two sides. 

 Deiter's cells. The cells in Corti's organ which support the outer hair cells. 

 delamination. The splitting of a cell layer into two or more layers of cells. 

 demersal eggs. Eggs (of fishes) which sink to the bottom of the water. 

 demilunes. Crescent-shaped cells of mucus-secreting acini, 

 dendrite. One of the branched processes of a neuron which carries impulses towards 



the cell body. 

 dentary. The membrane bone of the lower jaw to which teeth are attached, 

 dentine. The bone-like substance which forms most of the material of a tooth. 

 dentition. The kind, number, and arrangement of the teeth. 

 depressor. A muscle which lowers an organ. A nerve which inhibits action. 

 dermatome. That portion of the epimere which forms corium. 



deuterencephalon. That part of the embryonic brain which includes mid- and hind- 

 brain. 

 deuterostomia. Animals in which the blastopore forms the anus or lies near the anus. 

 deutoplasm. The passive nutritive portion of the germ-cell. 

 development. The process by which an egg changes into an adult. 

 diabetes. A disease which is marked by excessive excretion of urine. 

 diaphragm. The muscular partition which in mammals separates thorax and abdomen, 

 diapophysis. A process of the neural arch which articulates with the tubercle of a 



rib. 

 diastema. A space between the teeth, especially between the canine and the lateral 



teeth. 

 diencephalon. The second of the five successive divisions of the brain. 

 differentiation theory. The theory that compound teeth have evolved by differentia-' 



tion from simple conical teeth. 

 digestion. The process by which foods are made soluble. 

 dioecious. Having sexes separated in two individuals. 

 diphyodont. Having two sets of teeth. 



diploe. The cancellous bone between the two layers of compact bone of the cranium. 

 diploid. The double number of chromosomes characteristic of body cells. 

 diplospondyly. The condition of double centra in vertebrae. 

 distal. Opposed to proximal. 

 dorsal. Pertaining to the back or dorsum. 

 duodenum. The anterior portion of the small intestine. 

 dura mater. The tough outermost membrane which surrounds the central nervous 



system. 



ectoderm. The outermost germ layer. , 



ectosarc. The outer layer of the Protozoon cell. 



effector. The end-organ, muscle or gland, which responds to nervous stimulation. 



efferent. Away from a center or organ. 



egg. The animal ovum. 



ejaculatory duct. The duct which unites the seminal vesicle and ductus deferens with 



the urethra. 

 "elan vital." Bergson's name for the "vital factor." 

 endobranchiate. Forms with endodermal gills. 

 endocardium. The epithelial membrane which lines the heart, 

 endochondral bone. Bone which develops within cartilage. 

 endocranium. The dura mater of the brain. 

 endocrine gland. A gland which secretes internally into the blood. 



