GLOSSARY OF TECHNICAL TERMS xxi 



CYTOTAXIS (Gr. KVTOS, cell ; raft?, arrangement), a movement or 



arrangement of cells in response to stimulation by other 



cells. 



CYTOTROPISM (Gr. KVTOS, cell ; 17)07717, turning), see cytotaxis. 

 DEGENERATION, simplification of structure by reduction of parts. 

 DENDRON (Gr. btvbpov, tree), a branching outgrowth of a nerve 



cell. 



DENTITION (Lat. dens, tooth), tooth arrangement. 

 DENUDATION (Lat. denudare, to lay bare), the wearing away of 



rocks by atmospheric and other agencies. 

 DERMATOGEN (Gr. Stpua, skin ; rt. ytv-, origin), the embryonic 



layer which, in higher plants, gives rise to the epidermis. 

 DETERMINANTS, hypothetical bodies of ultramicroscopical size, 



which are supposed to exist in the chromatin substance of 



the nucleus and to be responsible for the transmission of 



inheritable characters. 

 DEUTOPLASM (Gr. Sevrepos, secondary ; ?rAao>ia, formative material), 



the food material or yolk stored in an egg. 

 DEVONIAN, the name given to the geological period in which the 



old red sandstone was deposited. 

 DIATOMS (Gr. 819, double; aro/xos, lit. indivisible, used in the 



sense of " atom "), a group of unicellular plants provided 



with double siliceous envelopes. 

 DICHOGAMY (Gr. St'xa, in two; ya/xos, marriage), maturation of 



the stamens and pistil of a flower at different times, prevent- 

 ing self-fertilisation. 

 DIHYBRIDISM (Lat. duo, two ; hybrida, hybrid), a Mendelian term 



for a cross between organisms which differ as regards two 



pairs of contrasted characters. 



DIMORPHIC (Gr. 6t'y, double ; popfai, form), having two forms. 

 DIOZCIOUS (Gr. 8ty, double ; oUiov, dwelling place), having the 



male and female organs in separate individuals. 

 ECHINOIDS (Gr. extuo?, hedgehog ; etSo?, form), sea-urchins. 

 ECTODERM (Gr. CKI-OS, outside ; 5ep/ua, skin), the outer layer of 



cells in a multicellular animal. 

 ECTOPLASM (Gr. e/cros, outside ; -nXa-v^a, formative material), the 



outer part of the protoplasm in cells. 

 EDENTATES (Lat. edentare, to knock out the teeth), a group of 



mammals without teeth in the front part of the jaw. 

 EMBRYO (Gr. eju/3puoi/), a young organism in an early stage of 



its development. 



