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GLOSSARY 



DARK-FIELD RING - an orange-yellow colored 

 illuminated ring as opposed to the silvery- 

 white surface of the sea urchin egg as des- 

 cribed by Ruunstrom (1928) under dark 

 field illumination. Not related to echino- 

 chrome. May be the area which is invag- 

 inated during gastrulation. 



DEDIFFERENTIATION - process of giving up 

 specialized characters and returning to the 

 more primitive (embryonic) conditions, 

 supposedly regaining the original and wider 

 range of potencies. Manifestation of powers 

 of cell adaptation to an abnormal environ- 

 ment such as in tissue culture, not normally 

 found in the living organism except possibly 

 in the blastema of regenerating tissue. The 

 existence of this change in cell structure 

 and function now questioned. Syn. , cata- 

 chony and einschmelzung. 



DEFLECTION - when dedifferentiated cells re- 

 main unable to redifferentiate, lying outside 

 the area determined by the term modulation. 

 Cells turned away from the line of normal 

 ontogenesis (Kasahara, 1935). 



DEGROWTH - actual reduction in mass subse- 

 quent to prolonged period of growth, prob- 

 ably indicating greater catabolic than ana- 

 bolic processes. Follows inanition. 



DELAMINATION - separation of cell layers by 

 splitting, a process of mesoderm forma- 

 tion. 



DETERMINANT - a Weismanian concept of a 



corpuscular unit which determines the qual- 

 ities and actions of cells in which it is con- 

 tained. Determinants possess powers of 

 growth and propagation and together con- 

 stitute the germ plasm. This concept sug- 

 gests that histological differentiation is 

 brought about by differential division until 

 a single determinant is left within the cell. 



DETERMINATION - a process of development 

 indicated when a tissue, whether treated 

 as an isolate or a transplant, still develops 

 in the originally predicted manner; the fix- 

 ing of fates or final assignments of parts of 

 the embryo at definite ontogenetic tinne; the 

 firm capacity of a tissue for self-differen- 

 tiation from which it cannot be deterred, 

 no matter what its environment, within 

 viable limits. An embryological rather 

 than a genetic concept. (Harrison, 1933. 

 Am. Nat. 67:306) 



DETERMINATION, DYNAMIC - opposed to in- 

 duction and refers, for example, to the 

 tendency of the marginal zone to invaginate 

 even when transplanted. (Vogt, 1925. ) 

 Formative movements. 



DETERMINATION, FIELD - state of organiza- 

 tion within an embryonic area probably 

 independent of the substrate; field of action 

 (Wirkungsfeld) or province of action 

 (Wirkungskreis) of Weiss (1925). (See 

 Field.) 



DETERMINATION, LABILE - definite but not 

 irrevocably fixed ability of tissue exposed 

 to inductive influences to continue develop- 

 ment in the induced direction even though 

 isolated as fragments. Syn. , competence, 

 or Bahnung. 



DETERMINATION, MATERIAL - formative 



movements which result in histological dif- 

 ferentiation. 



DETERMINATION, NEGATIVE - lack of certain 

 essential ingredients within the blastomere 

 necessary to the formation of a complex 

 embryo (e. g. , blastomere "D" in Dentalium 

 and Tubifex). 



DETERMINATION, PROGRESSIVE - determina- 

 tion in time rather than in space, advancing 

 from the more general to the more specific. 



DETERMINATIONSGESCHEHEN - all of the in- 

 visible processes in a blastema (and its 

 vicinity) which determine the morphogenesis 

 of the region. These processes may in- 

 volve two phases, those of self-organization 

 and those of segregation. (Lehmann, 1942. ) 

 Syn. , determinator system or realisator- 

 system (Lehmann, 1942). 



DEUTENCEPHALON - caudal region of the 



brain which later forms the nnesencephalon, 

 metencephalon and myelencephalon. 



DEUTEROKY - reproduction of both sexes from 

 parthenogenetic eggs (see arrhenotoky and 

 thelytoky). 



DEUTOPLASM - yolk or secondary food sub- 

 stance of the egg cytoplasm, non-living. 



DEVELOPMENT - gradual transformation of de- 

 pendent differentiation into self-differentia- 

 tion; transformation of invisible multiplicity 

 into a visible mosaic; elaboration of com- 

 ponents in successive spatial hierarchies. 



DEVELOPMENT, MOSAIC - "all the single 

 primordia stand side by side, separate 

 from each other like the stones of a mosaic 

 work, and develop independently, although 

 in perfect harmony with each other, into 

 the finished organism. " (Spemann, 1938). 

 Some believe there is pre -localization of 

 embryonic potencies within the egg, the test 

 for which would be self-differentiation. 



DEVELOPMENT, REGULATIVE - type of devel- 

 opment requiring organizer or inductor in- 

 fluences since each of the early blastomeres 

 could develop into whole embryos. Struc- 

 tures are progressively determined through 

 the action of evocators. 



DIAPAUSE - a normal state of dormancy in the 

 development of some animals (e. g. , insects) 

 not to be confused with hibernation because 

 this condition is independent of any environ- 

 mental factors. 



DICEPHALUS TETRABRACHINUS - condition 



attained when the first furrow of the amphib- 

 ian egg coincides with the sagittal plane and 

 the constriction is exaggerated, resulting 

 in duplications of the chorda, auditory ves- 

 icles, and fore-limbs. 



