 Gestation: carrying one or more embryos in uterus.  Humans: 266 days  Rodents: 21 d  Dogs: 60 d  Cows: 270  Elephant: 600 days

 Conception: fertilization of egg by sperm  Humans: occurs in oviduct  Contact of sperm with egg surface: metabolic rxns within egg that trigger the onset of embryonic development.  Enzymes in acrosome of sperm dissolve outer egg membrane. (lock and key fit: sperm to egg)  Fast block to polyspermy: change in memb. Potential  Slow block: cortical rxn: enzymes released from cortical granules to outside (exocytosis).

 Different species differ in the stage of meiosis the egg has reached at fertilization.  Some: have completed meiosis at time of release from female  Humans: arrested at metaphase of meiosis II. Until fertilized.

 Sperm requires about 6 hours in female reproductive tract … before ready to encounter egg.  Egg: is cloaked by follicle cells (released with it during ovulation)

 HCG: human chorionic gonadotropin: from embryo.  Acts like LH  Maintain secretion of progesterone and estrogens by the corps luteum.  HCG levels so high they go into urine (detected in pregnancy test)

 Contains blood vessels of both  Mother  Embryo

 Organisms development determined by  Genome of zygote  Neighboring cells (who directs the first differentiations?)

 Early embryonic cells become different because of  Cytoplasmic determinants: maternal substances found in Unfertilized egg: uneven distribution  Heterogeneous cytoplasm of polarized egg (mRNA, proteins, mitochondria)  Location in embryonic regions with different characteristics. Inner layer or trophoblast  Induction: cell stim by nearby cells (gene expression)  Diffusible chemicals OR cell surface interactions

 An animals spatial organization: where organs and limbs are found  Control by induction from nearby tissue

 Cell differentiation: through selective control of Gene Expression.  Cell signaling: Timely communication of instructions telling cells what to do and when to do it.  Morphogenisis: process by which animal takes shape. Differentiated cells end up where they are supposed to.

 Cells developmental potential: range of structures it can give rise to.  Goes down with cells “age”  Mammals: zygote cells are totipotent until the 8 cell stage.

 Cleavage : 24 hours post conception: zygote begins to divide (without dbl cytosol)  Blastocyst (blastula), sphere of cells containing a cavity: made by cleavage(1 wk. post conception). Implants in uterus. Trophoplast: outer layer  Gastrulation : produces 3 layered gastrula  Organogenesis: rudimentary organs from which adult structures arise.

 3 layers  Endoderm: lines tracts, organs  Mesoderm: muscle, skeleton  Ectoderm: skin, CNS

 Notochord: from dorsal mesoderm: signals the ectoderm to form Neural plate.  Neural plate: dorsal ectoderm:  becomes nueral tube. Becomes CNS  In only vertebrates: have neural crest:  become, peripheral nerves, teeth skull bones etc.

 After completion of neural tube:  Mesoderm is making 2 tissue layer of coelom.  Somites are beside notochord: they will become segmented: muscle and vertebrae.

 Adaptations are needed to avoid drying out on land (for all amniotes: reptiles and mammals) amniotes: extraembronic membranes (chorion, amnion, yolk sac)  1. shelled egg of birds and reptiles  And monotremes (mammals that lay eggs- platypus)  2. uterus (eutherians)  Marsupials  mammals

 Changes in cell Shape and Position  Cell shape: ie neural plate forms neural tube  Reorganize cytoskeleton. Actin fibers contract.  Cell position:  Cells “crawl” within the embryo with cytoskeletal fibers extend and retract.

 Extracellular matrix: secreted glycoproteins outside the plasma membrane.  Direct migrating cells  May act as tracks  Provide anchorage