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Embryology (Cleavage and formation of blastocyst (Implantation…

- - - - Inner cell mass is animal pole as becomes animal,
        Opposite pole referred to as 'vegetal pole'
      - Trophoblast is the outer superficial layer.
        
        Wall of blastocyst, forms foetal (opposed to maternal) portion of placenta
  - - - Inner cell mass differentiates into two layers, hypoblast is a lower layer of cuboidal/columnar epithelial cells, epiblast is upper layer of cuboidal/columnar cells.
    - - Epithelial cells in roof of this cavity form amnion (foetal membrane)
    - - Formed by migration of cells at edge of hypoblast moving to cover inner surface of blastocyst wall.
      - With hypoblast this membrane forms wall of yolk sac
        
        Layer of cells derived from yolk sac begin to proliferate between it, the amnion and trophoblast. Form loose connective tissue (mesenchyme) and called extraembryonic mesoderm
        
        several cavities develop within mesoderm and fuse forming extraembryonic coelom The extraembryonic mesoderm fused to chorion and amnion so coelom forms choronic cavity. Band of extraembryonic mesoderm connects the embryonic disc to chorion : connecting stalk
    - - Rearrangement and migration of epiblast cells of bilaminar disc
      - Establishes body plan of embryo
      - Bilaminar -> Trilaminar
      - Formation of primitive streak (groove on dorsal surface of epiblast). Formed by invagination of epiblast, elongates from caudal to cranial embryo establishing head tail and head ends
      - Primitive node formed at head end of primitive streak (node is rounded structure of epiblastic cells)
      - Epiblast cells remaining on dorsal surface are ectoderm
      - Faint depression forms on dorsal surface of ectoderm. The oropharyngeal membrane
      - Hypoblast cells now called endoderm
      - Mesenchyme displaces hypoblast, this layer called mesoderm
      - Neuralation
        
        Occurs simultaneously as gastrulation