GLOSSARY 465 



GASTHULATION - dynamic prooeeaeB Involving cell movements which change the embryo from a monodermlc to a 

 di- or trl-dermlc form, generally involving Inward movement of cells to form the enteric endoderm. 

 Process varies In detail in different forms, but may include epiboly, concrescence, confluence, in- 

 volution, invagination, extension, convergence - all of which are descriptive terms for morphogene- 

 I, tic movements. 



GEFALLE - a continuous, quantitative gradation of a definite condition within a cell colony. (See gradient) 



GEL - a system in which there is a reduction in the amount of solvent relative to the amount of solid sub- 

 stance, thereby causing the whole to become viscous (e.g., asters). 



GENE - self-producing molecule transmitted by the chromosome which determines the development of the 

 characters of the individual, some of which may be solely embryonic. 



GENETIC LIMITATION - each cell must react exclusively in accordance with the standards of the species 

 which it represents. 



GENOME - haplold gene coii5>lex; minimum (haploid) number of chromosomes with their genes derived from a 

 gamete. 



GENOTYPE - the actual genetic make-up of an individual, regardless of its appearance (opposed to phenotype) 



GEOTONUS - position correct in respect to gravity. 



GEEM - the egg throughout its development, or at any stage. 



GEHM BAND - distinguishable bands of material in the (Molluscan) egg which will give rise respectively to 

 ectoderm, endoderm, and mesoderm of the embryo. 



GEPM CELL - a cell capable of sharing In the reproductive process, in contrast with the somatic cell. 

 (E.g., spermatozoon or ovum.) Syn. gamete. 



GEHM LAYKR - a more-or-less artificial spatial and histogenic distinction of cell groups beginning in the 

 gastrula stage, consisting of ectoderm, endoderm and mesodermal layers. No permanent or clear cut 

 distinctions, as shown by transplantation experiments. 



GEEM RING - ring of cells which show accelerated mitotic activity, generally a synonym for the marginal 

 zone which becomes the lips of the blastopore. The rapidly advancing cells in epiboly. (Syn., 

 marginal zone) 



GERM WALL - advancing boundary of the (chick) blastoderm including syncltia and the zone of Junction. 



GERMINAL LOCALIZATION - every area of the blastoderm (or of the unfertilized egg) corresponds to some 



future organ. Unequal growth produces the differentiation of parts (His, iS?**). This concept led 

 to the Mosaic Theory of Roux. (See fate map.) 



GERMINAL VESICLE - the pre-maturatlon nucleus of the egg. 



GERONTOMORPHOSIS - phylogenetlc effects produced by modifying characters which are present In the line 

 of adults. 



GESTALTEN - a system of configurations consisting of a ladder of levels; electron, atom, molecule, cell, 

 tissue, organ, and organism, each one of which exhibits specifically new modes of action that cannot 

 be understood as mere additive phenomena of the previous levels. With each higher level new con- 

 cepts become necessary. The parts of a cell cannot exist Independently, hence the cell is more than 

 a mere aggregation of its parts, it is a patterned whole. Coherent unit reaching a final configura- 

 tion in space (W. Kohler). Gestaltung means formation. 



GESTATION - period of carrying the young (mammal) within the uterus. 



GIBBS-THOMPSON LAW - solidification from the accumulation of surface-acting substances by the lower sur- 

 face tension at the surface of a drop (or cells), causing the potential energy of the combined 

 system (liquid drop immersed within another liquid drop with which It is not mlscible) to drop to 

 a minimum. 



GONOCHORISM - development or history of sex differentiation (Haeckel). Opposed to hermaphroditism. 



GONOMEHY - continued separation during cleavage of the chromosome sets from sperm and from egg in hybrid 

 crosses. Theory that the maternal and paternal chromosomes remain apart throughout development. 



GRADIENT - gradual variation along an axis, scaled regions of preference, two-dimensional pattern. 

 (gefalle of Boveri.) (See writings of Child.) 



GRADIENT, ACTrVITV - gradient established with appearance of the grey crescent in the amphibian egg, 

 extending dorso-ventrally across the equator. 



GRADIENT, AXIAL - metabolic gradient determined by differences in electrical potential or by experiments 

 demonstrating differential susceptibility (e.g., to KCN) . 



GRADIENT CONCEPT - Idea of physiological polarity indicated when an individual (e.g., Planarla) is trans- 

 sected and each fragment reproduces the missing portions while retaining the original polarity; any 

 two-dimensional concentration gradient as shown, for Instance, by anlmalizing or vegetalizing factors 

 In early morphogenesis (Ruunatrom). 



GRADIENT, INHIBITION - refers to the balance of animal and vegetal hemisphere gradients In the ( sea urchin) 

 egg (Ruimstrom) which gradients are actually antagonistic to each other and yet both are necessary 

 for normal (balanced) development. 



GRADIENT, PIGMENT - when pignent Is present it is generally concentrated at the centers of greatest meta- 

 bolic activity. 



GRAFT - a portion of one embryo removed and placed either among the tissues (a transplant) or the membranes 

 (e.g., chorlo-allantoic graft) of another embryo. 



GRAFT, CHOHIO-ALLANTOIC - method of growing a graft on the extra-embryonic membranes of the chick, the 

 membranes reacting to the local irritation of a (foreign) graft In such a manner as to surround It 

 with a richly vascular tunic of indifferent tissue, rich in the requisites for survival, growth, and 

 differentiation of the graft. The graft is never incorporated as a transplant by the host itself. 

 Graft on the chick chorlo-allantols. 



GRAFT KfBBID - organism formed from host and graft, showing characteristics of both stocks. 



