Primitive streak

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(Anat.) an opaque and thickened band where the mesoblast first appears in the vertebrate blastoderm.

See also: Primitive

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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An alternative theory suggests monozygotic conjoined twins occur when splitting of the embryo is incomplete at the primitive streak stage of development (Kaufman, 2004).
The PASE structures even exhibit the earliest signs of initiating a "primitive streak," although it did not fully develop.
The brutal crimes visited on Angelique Charlotte Bulatao show a primitive streak bubbling out of the muck and bursting through the surface calm.
Other projects by Storey include Catalytic Clothing, garments and textiles using photocatalysts to improve air quality; Wonderland, exploring eco-friendly alternatives to plastic for consumer products; and Primitive Streak, a clothing collection developed with her sister, biologist Kate Storey, that draws inspiration from the first 1,000 hours of human life.
That particular time point was supposedly adopted because in the next stage of development, the embryo forms the primitive streak, the first sign of a rudimentary nervous system, and is also supposedly past the point at which twinning can occur.
(5) It is thought that this OEIS complex results from a single defect of early blastogenesis or defect of a caudal mesodermal migration during primitive streak period that later contributes to the formation of infra umblical mesenchyme, cloacal septum and vertebrae.
According to Jubb and Kennedy (1970) and Noden and Dalhunta (1985) conjoined twins are non-inherited teratological defects and monozygotic, thus monstrosities arise due to incomplete embryonic division into components usually at primitive streak development stage.
The majority of congenital defects are due to multifactorial etiology such as genetic or environmental (infectious diseases viruses drugs poisonings plants mineral salts and vitamin (A D E) deficiency) which cause the improper separation of primitive streak after day 13 of fertilization (El Sheikh et al.
Specifically, cell differentiation begins dramatically and the 'primitive streak'--a cluster of cells from which the brain, nervous system, and organs of the body will grow--appears.
They then used time-lapse microscopy to follow the cells in what has long been called "polonaise movements," the large-scale travels of ectoderm cells which lead to the formation of mesoderm along the midline where the primitive streak forms.