. The elements of experimental embryology. Embryology, Experimental; Embryology. 68 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY side. Further, in sub-lethal concentrations, the dorsal regions are the most inhibited in their differentiation.^ This last method allows us to make a further statement, namely that the dorso-ventral activity-gradient becomes progressively more intense (steeper) between fertilisation and gastrulation. In just- fertilised eggs, disintegration in lethal concentrations begins at the animal pole and then spreads along the dorsal side: in some cases a second centre of disi
. The elements of experimental embryology. Embryology, Experimental; Embryology. 68 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY side. Further, in sub-lethal concentrations, the dorsal regions are the most inhibited in their differentiation.^ This last method allows us to make a further statement, namely that the dorso-ventral activity-gradient becomes progressively more intense (steeper) between fertilisation and gastrulation. In just- fertilised eggs, disintegration in lethal concentrations begins at the animal pole and then spreads along the dorsal side: in some cases a second centre of disintegration appears in the region of the grey. Fig. 28 Differential susceptibility in a frog's egg exposed to KCN from the 2-cell stage: disintegrated cells are shown light. The animal pole area (central) has disintegrated; also an area of cells, near the equator on one side, in the future organiser region. (After Bellamy, Biol. Bull, xxxvii, 1919, modified.) crescent before the primary disintegration has spread to this area. During cleavage, the susceptibility of the dorsal lip region in- creases, until in late blastulae this region begins to disintegrate before or at the same time as the apical pole. In gastrula stages, the dorsal lip region is always the first to disintegrate.^ It is probable that this process is correlated with the acquisition of organiser properties by the dorsal (grey crescent or dorsal lip) region (fig. 28). The method by which bilateral symmetry is determined in the egg of Echinoderms is still problematical-; but the localisation of the plane can be revealed by susceptibility experiments at a stage before any bilateral symmetry is visible in the embryo.^ Further, a labile determination of bilateral symmetry has been ^ Bellamy, 1919; Bellamy and Child, 1924. 2 Horstadius, 1928. ^ Child, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these ill
Size: 1618px × 1545px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversi, booksubjectembryology
