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Gastrulation
 Sorts the cells of the blastula into 3 distinct layers
 Cells on 1 part of the blastula begin to burrow inwards forming the blastopore
 This burrowing produces an external lip—the dorsal lip of the blastopore—which
becomes the organizer for the gastrula
 Cells on the surface of the blastula roll over the lip and move into the interior of
the gastrula
 Cells migrate along the dorsal wall of the blastocoel and become organized into
mesoderm and endoderm
 Within the endoderm the archenteron forms (will form the gut)
 The blastopore lip enlarges to form a complete circle which surrounds the yolk
plug (will become the anus)
 Ectoderm forms the cells remaining on the surface
Differentiation
 By the late gastrula stage a cells developmental fate is fixed
 Heterogeneous distribution of cytoplasmic contents leads to regional differences
the zygote
 During cleavage cells receive different substances (mRNA, proteins, etc)
 Gene expression in and the fate of cells are influenced by the cytoplasmic
contents and by interactions among cells
Determination
 Progressive restriction of a cells developmental potential
 After a certain time, even though a cell has all the genes present in an organism,
its fate cannot be changed by moving it into another region of the embryo

Organogenesis
 Germ layers give rise to early organs
 First evidence is folds, splits, and condensation of cells in the layers
 First organs to develop in chordates are the neural tube and notochord (organizer
for the vertebrae and muscles)
Neural tube formation
 The dorsal mesoderm above the archenteron condenses to form the notochord
 The ectoderm above the notochord thickens to form the neural plate which sinks
into the mesoderm and rolls up to form the brain and spinal cord
 The notochord elongates and stretches the embryo lengthwise
 The mesoderm cells that form the vertebrae condense around the notochord
 More mesoderm condenses along the sides of the notochord forming somites—
blocks of tissue that will form the muscles
Morphogenesis
 Specific changes in cell shape, position, and adhesion that shape the embryo
 Tissues are created by cells “crawling” around using amoeba-like movements and
the extra-cellular matrix as a guide
 Cells can secrete substances to promote or inhibit other cells to migrate
 Cells can receive signals with surface receptor proteins
Pattern formation
 Development of an animal’s spatial organization along 3 axes
 Proximal-distal
 Anterior-posterior
 Dorsal-ventral
 Cells can respond to chemical or physical signals to know their location relative
to other cells within the embryo
Fate of the germ layers
 Ectoderm: epidermis, epidermal glands, inner ears, eye lenses, brain and nervous
system
 Mesoderm: muscles, skeleton, kidneys, sex organs, circulatory system
 Endoderm: digestive tract lining, liver, pancreas, lungs