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Cleavage, Gastrulation

Cleavage, Gastrulation. & formation of the germ layers. Cleavage. Fertilization is followed by cleavage, a period of rapid cell division without growth Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres.

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Cleavage, Gastrulation

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  1. Cleavage, Gastrulation & formation of the germ layers.

  2. Cleavage • Fertilization is followed by cleavage, a period of rapid cell division without growth • Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres

  3. The eggs and zygotes of many animals, except mammals, have a definite polarity • The polarity is defined by distribution of yolk, with the vegetal pole having the most yolk

  4. LE 47-9 Zygote 0.25 mm 2-cell stage forming 4-cell stage forming Eight-cell stage (viewed from the animal pole) 8-cell stage 0.25 mm Animal pole Blasto- coel Blastula (cross section) Vegetal pole Blastula (at least 128 cells)

  5. Gastrulation • Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut

  6. Gastrulation • The three layers produced by gastrulation are called embryonic germ layers • The ectoderm forms the outer layer • The endoderm lines the digestive tract • The mesoderm partly fills the space between the endoderm and ectoderm

  7. LE 47-12 CROSS SECTION SURFACE VIEW Animal pole Blastocoel Dorsal tip of blastopore Dorsal lip of blastopore Vegetal pole Blastula Blastocoel shrinking Archenteron Ectoderm Mesoderm Blastocoel remnant Endoderm Key Future ectoderm Future mesoderm Yolk plug Yolk plug Gastrula Future endoderm

  8. During gastrulation, some epiblast cells move toward the blastoderm’s midline and then detach and move inward toward the yolk

  9. LE 47-13 Epiblast Primitive streak Future ectoderm Endoderm Migrating cells (mesoderm) Hypoblast YOLK

  10. Organogenesis • During organogenesis, various regions of the germ layers develop into rudimentary organs

  11. Many structures are derived from the three embryonic germ layers during organogenesis

  12. Summary • At completion of cleavage, the blastocyst forms • The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the blastocyst forms a flat disk of cells • As implantation is completed, gastrulation begins • The extraembryonic membranes begin to form • By the end of gastrulation, the embryonic germ layers have formed

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