Embryonic Development and Body Cavities

  • Symmetry in Animals

    • Importance of symmetry in animal development.

  • Embryonic Tissue Layers

    • Ectoderm (outer layer)
    • Develops into body coverings (skin, scales, feathers) and components of the nervous system.
    • Mesoderm (middle layer)
    • Forms muscles, circulatory system, reproductive system, and more.
    • Endoderm (inner layer)
    • Develops into the lining of the digestive tract and some internal organs (lungs, liver).

  • Gastrulation

    • A process that leads to the formation of tissue layers in an embryo.
    • Key structures at the end of gastrulation:
    • Two or three tissue layers present.
    • Formation of the blastopore (opening for cell movement into the archenteron).
    • Blastopore: The opening of the cell that allows for the formation of internal structures.
    • Archeteron: The space that will develop into the gut.

  • Body Cavities

    • Definition of coelom: A body cavity that forms within the mesoderm, not part of the digestive tract.
    • Types of Body Cavities:
    • Coelomates: True coelom formed within the mesoderm.
    • Pseudocoelomates: Coelom formed between the endoderm and mesoderm, not surrounded by mesoderm.
    • Examples:
    • Nematodes vs. annelids have different formations; one is a pseudocoelom (nematodes) while the other has a true coelom (annelids).

  • Embryonic Development Stages

    • Sequence of events:
    1. Fertilization
    2. Cleavage (cell division, no growth in overall size)
    3. Formation of blastula (hollow ball of cells)
    4. Gastrulation (cells migrate to form tissue layers)
    • Differentiation into protostomes and deuterostomes:
    • Protostomes: Mouth forms from the blastopore.
    • Deuterostomes: Anus forms from the blastopore; mouth forms later.

  • Determinacy in Cell Fate

    • In protostomes, the fate of cells is determined by the eight-cell stage.
    • In deuterostomes, cells maintain indeterminate fates, allowing for potential development into whole organisms regardless of cell removal.

  • Coelom Formation:

    • Protostomes: Mesoderm forms near the blastopore and separates to create the coelom.
    • Deuterostomes: Mesoderm originates from the opposite end of the blastopore and invaginates to form the coelom.

  • Phylogenetic Signal:

    • Traits identifiable in animals based on common ancestry:
    • Bilateral symmetry.
    • Presence of three tissue layers.
    • Importance in identifying evolutionary relationships among animals.
    • Hox genes: Shared among bilaterally symmetrical animals influencing body plan and development.

  • Acoelomates:

    • Animals lacking a coelom, for example, flatworms.
    • Contrast with coelomates and pseudocoelomates in evolutionary significance.