Variability of quantitative morphogenetic parameters during early morphogenesis of the loach, <Emphasis Type="Italic">Misgurnus fossilis</Emphasis> L.

Analysis of normal variation in quantitative morphological characters during the early embryonic development of the loach, based on observations on individual developmental trajectories of living embryos, shows that the dorsoventral differentiation of the blastoderm proceeds in two stages. Initially, at the onset of epiboly, the sagittal (short) and transverse (long) blastoderm meridians are marked off, and only then, upon germ ring (GR) formation, differentiation between the opposite poles of the sagittal meridian takes place. The embryonic shield (ES) usually appears in the segment of the blastoderm where the radius of its external curvature reaches a maximum and, therefore, the active surface tension at the blastoderm boundary with the YSL periblast) and yolk is the highest. In this case, the convergence of inner cells toward the future dorsal segment (leading to ES formation) is a mechanical consequence of surface tension anisotropy. The normal course of epiboly is associated with periodic changes in the curvature of the blastoderm external surface, with new structures (the dorsal segment, GR, and ES) are marked off only when the surface curvature becomes maximally uniform. Although the ES in most embryos appears within the initial dorsal segment, individual developmental trajectories have been traced where the GR starts to form at the dorsal pole of the blastoderm but the ES develops on its opposite site, at the point of GR closure. In both cases, GR formation is initiated at the point of convergence of centrifugal cell migration flows that arise in the marginal zone of the blastoderm upon GR initiation or closure.