The origin of vertebrates and their symmetry, segmentation, chord and tubular nervous system

Abstract

Development 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.