Abstract
AbstractDevelopment of vertebrata begins with formation of a multicellular organism by ordered repeated division of a reproductive cell and nondisjunction of the new formed cells, which have kept connection by means of the extracellular matrix. Further there is a consecutive formation of organisms due to aggregation of similar structures: blastaea; segmentella, supersegmentella. Supersegmentella gave development to tunicates, hemichordates, chordates like lancelet and to predecessors of vertebrata. Segmentation of organisms is determined by aggregation of supergastraeas into one integrated. Symmetry is determined by structure-forming quality of extracellular matrix. Symmetry of primary organisms was radial; then radial-bilateral, and the first plane of symmetry divided the organism into dorsal and ventral sides. With the arrangement of supergastraeas in a line radial-two-plane symmetry consistently formed. Radial-three-plane symmetry formed by association of two segmentellas by posterior edges. The third plane of symmetry divided the organism into anterior and posterior antimeres. From extracellular matrix originated mesogloea, and then a chord; endodermic embolies gave development to the primary gut; ectodermic embolies after the concentration there earlier diffusely located nervous cells transformed first into a trench, and then into a tubular nervous system; the condensed nervous fabric of aboral poles gave development to the central nervous system. The glandulocytes of supergastraeas became starting material for all glands of the organism.
Highlights
The origin of vertebrates remains an unresolved question in biology
Cells and tissues of multicellular organisms adapted themselves to the changing environment, became specialized and localized in the body so as to conform to its intrinsic symmetric organization at any given stage of evolution
Mesoglea that had played an important role as an organizer of primary multicellular animals throughout the long period of previous evolution transformed into the chord to further govern the development of chordate animals
Summary
Based on a comparison of the structure and development of animals and on their own conjectures, anatomists have suggested a large number of concepts of the origin of Bilateria, including vertebrates. These hypotheses and their analysis are presented in monographs (Bresslau and Reisinger 1933; Hyman 1951; Willmer 1994; Iordansky 2001; Saveliev 2005), theses (Gorodilov 2002), and reviews (Arendt and NüblerJung 1999; Erwin, 1999; Malakhov 2004; Gerhart et al 2005; Gerhart 2006).
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