Stages of Animal Development

• Stages of Animal Development

  1. Gametogenesis
  2. Fertilization
  3. Implantation
  4. Gastrulation
  5. Embryonic Stage
  6. Fetal Stage
• Gametogenesis

  • production of gametes via meiosis.
• Fertilization

  • results in a unicellular diploid zygote. Meiosis II of egg cell proceeds.
• Implantation

  • process where the blastocyst implants itself in the endometrium, signals the start of pregnancy.
• Gastrulation

  • formation of germ layers- Ectoderm, Endo, and Meso.
  • Gestation - carrying of the embryo inside the female reproductive tract, specifically the uterus. Can last up to 9 months in humans.
• Embryonic Stage

  • formation of 4 membranes that support, protect, and nourish the embryo. Stage of placenta and organ development.
• Fetal Stage

  • extends from the beginning of the ninth week after fertilization to about 38 weeks after fertilization.
• Early Development

  • The Zygote
  • Cleavage: From One Cell to Many
  • The Blastocyst
  • Implantation
  • Formation of Germ Layers
• The Zygote

  • is a fertilized egg that has the potential to give rise to all the diverse cell types of the complete individual.
  • In most organisms, the cytoplasm contains yolk, which serves as food for the developing embryo. Yolks are absent from the human zygote.
• Cleavage: From One Cell to Many

  • Cleavage is a process wherein the zygote undergoes rapid mitosis. By about 24 hours after fertilization, the human zygote has completed the first mitotic division and reached the two-cell stage.
  • At about the 16-cell stage, the embryo consists of a tiny cluster of cells called the morula. By the time the embryo reaches the uterus, on about the fifth day of development, it is in the morula stage.
• The Blastocyst

  • During this period, its cells arrange themselves into the form of a hollow ball called a blastocyst (blastula), which eventually forms nutritive membranes (the chorion and placenta) that surround the embryo.
• Implantation

  • Implantation of embryo in the endometrium (lining) of the uterus begins on the seventh day of embryonic development. During this period, enzymes destroy some tiny maternal capillaries in the wall of the uterus.
• Formation of Germ Layers

  • The cells of the inner cell mass of the blastocyst arrange themselves to form a two-layered disk.
  • The cells of the lower level then merge to line an inner cavity, the primitive gut, or archenteron, which will eventually develop into the digestive tract and certain other structures.
• Inner Cell Mass

  • Ectoderm - Nervous system, outer layer of skin (epidermis, nails, hair, etc.), Pituitary gland
  • Mesoderm - Skeleton, Muscles, Circulatory, Excretory, and Reproductive System, Inner layer of skin, outer layers of digestive tube
  • Endoderm - Lining of digestive tube and respiratory system
• Stages of the Human Life Cycle

  1. Embryo
  2. Fetus
  3. Neonate
  4. Infant
  5. Child
  6. Adolescent
  7. Young Adults
  8. Middle Age
  9. Old Age