Animal Development

• Zygote


  A fertilized egg cell that resulted from the union of the male and female gametes, has the potential to give rise to all the diverse cell types of the complete individual
• Yolk


  Found in the cytoplasm of most vertebrate zygotes, serves as food for the developing embryo, amount and distribution vary among different animal groups, absent from the human zygote
• Cleavage


  1. A process wherein the zygote undergoes a rapid mitosis, from one cell, many more cells are created (from one cell to many)

  2. About 24 hours after fertilization, the human zygote has completed the first mitotic division and reached the two-cell stage

  3. Each cell of the two-cell stage embryo undergoes mitosis, bringing the number of cells to four

  4. Repeated divisions continue, increasing the number of cells

  5. At about the 16-cell stage, the embryo consists of a tiny cluster of cells called the morula

  6. As it continuously takes place, the embryo is pushed along the uterine tube by ciliary action and muscular contraction

  7. Morula Stage - the embryo has reached the uterus on about the 5th day of development
• Blastocyst


  The cells arrange themselves into the form of a hollow ball, also called the blastula, chorion and placenta eventually form and surround the embryo
• Inner Cell Mass


  A little cluster of cells that projects into the cavity of the blastocyst, gives rise to the embryo
• Implantation


  Begins on the 7th day of embryonic development, the embryo is implanted in the endometrium of the uterus, Enzymes destroy some tiny maternal capillaries in the wall of the uterus, Blood from these capillaries comes in direct contact with the trophoblast of the embryo, temporarily providing a rich source of nutrition, Completed by the 9th day of development
• Formation of Germ Layers


  The cells of the inner cell mass arrange themselves to form a two-layered disk, The cells of the lower level then merge to line an inner cavity, the primitive gut (archenteron), These cells make up the endoderm, while the cells that remain to cover the embryo, becoming its outermost layer, form the ectoderm, A third layer of cells, the mesoderm, proliferates in between the two cell layers
• Germ Layers and their Derivatives

  • Ectoderm - Nervous system, Epidermis and its associated structures (nails, hair, etc.)
  • Mesoderm - Skeleton (bone and cartilage), Several organ systems (circulatory, excretory, and reproductive), Muscles, Dermis
  • Endoderm - Lining of digestives tubes, Lining of structures that develop from the digestive tube such as in the respiratory system, Outer layers of the digestive tube, Pituitary gland
• Stages in the Human Life Cycle

  • Embryo - From conception to end of 8th week of prenatal development
  • Fetus - Beginning of 9th week of prenatal development up to birth
  • Neonate - From birth up to 4 weeks of age
  • Infant - End of 4th week up to 2 years of age
  • Child - From age 2 years up to puberty
  • Adolescent - From puberty up to adult
  • Young Adult - From the end of adolescence up to about age 40 years
  • Middle Age - From age 40 years up to about age 65 years
  • Old Age - Age 65 up to death