U60 GLOSSARY 



DABK-FIELD RING - an orange-yellow colored Illuminated ring aa opposed to the silvery white surface of the 

 sea urchin egg as described by Ruunstrom (1928) under dark field Illumination. Not related to echlno- 

 chrome. ttey be the area which Is Invaglnated during gastrulatlon. 



DEDIFFEREnroiATION - 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 environment such aa in tissue culture, not normally found 

 in tie living organism except possibly in the blastema of regenerating tissue. The existence of thla 

 change in cell structure and function now questioned. Syn., catachony and einschmelzung. 



DEFLECTION - when dedifferentiated cells remain unable to redifferentlate, lying outside the area deter- 

 mined by the term modulation. Cells turned away from the line of normal ontogenesis (Kasahara, 1955). 



DEGHOWTH - actuql reduction in mass subsequent to prolonged period of growth, probably indicating greater 

 catabolic than anabolic processes. Follows inanition. 



DELAMINATION - separation of cell layers by splitting, a process of mesoderm formation. 



DETERMINANT - a Weismanlan concept of a corpuscular unit which determines the qualities and actions of 

 cells In which it is contained. Determinants possess powers of growth and propagation and together 

 constitute the germ plasm. This concept suggests 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 trans- 

 plant, still develops In the originally predicted manner; the fixing of fates or final assignments of 

 parts of the embryo at definite ontogenetic time; the firm capacity of a tissue for self-differentia- 

 tion from which it cannot be deterred, no matter what its environment, within viable limits. An 

 embryologlcal rather than a genetic concept. (Harrison, 1953' Am. Nat. 67:506) 



DETEJMINATION, DYNAMIC - opposed to induction and refers, for example, to the tendency of the marginal 

 zone to Invaglnate even when transplanted. (Vogt, I925.) Formative movements. 



DETEMINATION, FIELD - state of organization within an embryonic areas probably Independent of the sub- 

 strate; field of action ( Wirkungafeld) or province of action (Wirkungskreis) of Weiss (I925). (See 

 Field.) 



DETEBMINATION, LABILE - definite but not irrevocably fixed ability of tissue exposed to inductive influ- 

 ences to continue development in the induced direction even though isolated as fragments. Syn., com- 

 petence, or Bahnung. 



DETERMINATION, MATERIAL - formative movements which result in histological differentiation. 



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 Tublfex). 



DETERMINATION, PROGRESSIVE - determination in time rather than in space, advancing from the more general 

 to the more specific. 



DETERMINATIONSGESCHEHEN - all of the Invisible processes In a blastema (and its vicinity) which determine 

 the morphogenesis of the region. These processes may Involve two phases, those of self -organization 

 and those of segregation. (Lehmann, 19^*2.) Syn., determinatorsystem or reallsatorayatem ( Lehmann, 

 191+2 ) . 



DEUTENCEPHALON - caudal region of the brain which later forma the mesencephalon, metencephalon and 

 myelencephalon. 



DEUTEROKY - reproduction of both sexes from parthenogenetic eggs (see arrhenotoky and thelytoky). 



DEUTOPLASM - yolk or secondary food substance of the egg cytoplasm, non-living. 



DEVELOPMENT - gradual transformation of dependent differentiation Into self-differentiation; trans- 

 formation of invisible multiplicity Into a visible moaalc; elaboration of components In successive 

 spatial hierarchies. 



DEVELOPMENT, MOSAIC - "all the single primordla stand aide 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, I958). Some believe there Is pre-locallzation of embryonic 

 potencies within the egg, the teat for which would be aelf-differentiatlon. 



DEVELOPMENT, REGULATIVE - type of development requiring organizer or inductor Influences since each of 

 the early blastomeres could develop into whole embryos. Structxirea are progressively determined 

 through the action of evocators. 



DIAPAUSE - a normal state of dormancy in the development of some anlmala (e.g.. Insects) not to be con- 

 fused with hibernation because this condition is Independent of any environinental factors. 



DICEPHALUS TETRABRACHINUS - condition attained when the first furrow of the amphibian egg coincides with 

 the sagittal plane and the constriction Is exaggerated, resulting In duplications of the chorda, 

 auditory vesicles, and fore-limbs. 



DICHIRUS - partial duplication of digits in hand or foot, possibly inherited. A type of Polydactyly. 



DICHOTCMY, DIFFERENTIAL - embryonic segregation; capacity of embryonic cells for self-differentiation 

 becomes itself differentiated (Llllie, 1929). 



DIFFERENTIATING CENTER - area responsible for the localization and determination of various regions of 

 the embryo, resulting In harmonious proportioning of parts. 



DIFFERENTIATION - acquisition of specialized features which distinguish areas from each other; progres- 

 sive increase in complexity and organization, visible and invisible; elaboration of diversity 

 through determination leading to histogenesis; production of morphogenetlc heterogeneity. Syn., 

 dlfferenzlerung. 



DIFFERENTIATION, AXIAL - variations In density of chemical and often indefinable Inclusions in the 

 direction of one diameter of the egg, called the egg axis (see gradient). 



