Zygote: fertilized egg (1 cell), divides through mitosis
Embryogenesis: stages of development between fertilization
and birth
Processes of embryogenesis:
·
Fertilization: fusion of the gametes
·
Cleavage: series of mitotic cell divisions after
fertilization. The cytoplasm forms many small cells, called blastomeres, which
form a sphere (blastula)
·
Gastrulation: Blastomeres change their relative
positions; the three germ layers of the embryo form (endoderm, mesoderm,
ectoderm)
·
Organogenesis: Cells interact with one another
and rearrange themselves to produce tissues and organs
·
Metamorphosis: in some animals, larva become
sexually mature adult organisms
·
Gametogenesis: germ cell differentiation occurs
Epigenesis: the view that organs of an embryo are formed de novo (from scratch) each generation
Preformation: the view that organs are already present, in miniature form, within an egg and a sperm.
The Preformation theory is flawed n that it is unable to account for intergenerational genetic variations
Now, most biologists believe that instructions for the formation of the organism are present in a fertilized egg
Primary germ layers and early organs
Pander discovered germ layers: ectoderm, mesoderm and endoderm
Ectoderm: outer layer of embryo, produces epidermis, brain and nervous system
Mesoderm: generates blood, connective tissue, heart, kidneys, gonads, bones and muscles
Endoderm: inner layer of embryo, forms epithelium of digestive tube and associated organs
The four principles of Karl Ernat Von Baer
1. The general features of a large group of animals appear earlier in development than do the specialized features of a smaller group
2. Less general characteristics develop from the more general until finally the most specialized appear
3. An embryo does not pass through the state/formations of other organisms but instead separates itself from them
5
4. The embryo of a higher form never resembles an adult of another form, but only an embryo of its own form
Blastospore: dimple, marks fixture dorsal (top) side of embryo. Formed at the beginning of gastrulation.
The blastospore expands to form a ring. Cells that migrate through it become mesoderm
The endoderm cells are the large yolky cells in the vegetal hemisphere.
Notochord: rod of mesodermal cells in the most dorsal portion of the embryo. Signals ectodermal cells above it to form a tube and become the nervous system instead of forming the epidermis
Neural precursor cells elongate, stretch and fold into the embryo, forming the neural tube
Mesodermal cells near the neural tube and notochord are segmented into somites
Somites: precursors to the back, skeleton and spine
1. The general features of a large group of animals appear earlier in development than do the specialized features of a smaller group
2. Less general characteristics develop from the more general until finally the most specialized appear
3. An embryo does not pass through the state/formations of other organisms but instead separates itself from them
5
4. The embryo of a higher form never resembles an adult of another form, but only an embryo of its own form
Blastospore: dimple, marks fixture dorsal (top) side of embryo. Formed at the beginning of gastrulation.
The blastospore expands to form a ring. Cells that migrate through it become mesoderm
The endoderm cells are the large yolky cells in the vegetal hemisphere.
Notochord: rod of mesodermal cells in the most dorsal portion of the embryo. Signals ectodermal cells above it to form a tube and become the nervous system instead of forming the epidermis
Neural precursor cells elongate, stretch and fold into the embryo, forming the neural tube
Mesodermal cells near the neural tube and notochord are segmented into somites
Somites: precursors to the back, skeleton and spine
Sources: "Developmental Biology" Tenth Edition - Scott F Gilbert
