Cram sheet
30 points- The five stages, in order: fertilization → cleavage → gastrulation → organogenesis → axis and pattern formation. Memorize this order; the exam asks "which happens last." Watch Stages of Animal Development: Cleavage, Gastrulation, Organogenesis on YouTube, from Professor Dave Explains Watch Animal Development: We're Just Tubes - Crash Course Biology #16 on YouTube, from CrashCourse
- THE BIG ONE — germ layer fates. Ectoderm: skin epidermis, nervous system, sense organs (plus neural crest). Mesoderm: skeleton, muscle, heart, blood, blood vessels, gonads, kidneys, dermis of the skin. Endoderm: lining of the digestive and respiratory tracts, liver, pancreas, thymus, thyroid. Watch Germ layer derivatives | Behavior | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Trick within the trick: skin has two germ layers. The epidermis is ectoderm; the dermis is mesoderm (from somites). Watch Germ layer derivatives | Behavior | MCAT | Khan Academy on YouTube, from khanacademymedicine
- The acrosome is a digestive-enzyme sac in the sperm head. It dumps its enzymes by exocytosis to eat through the egg coats (jelly + vitelline envelope in sea urchins, zona pellucida + granulosa cells in mammals). Watch Acrosome Reaction in Sea Urchin on YouTube, from Hussain Biology
- Sea urchin sperm build an acrosomal process out of polymerized actin to reach the egg membrane. Mouse sperm have no acrosomal process — the whole head tunnels through the zona pellucida. Watch Acrosome Reaction in Sea Urchin on YouTube, from Hussain Biology
- Membrane fusion triggers a wave of free Ca²⁺ released from internal stores, starting at the sperm entry point and crossing the whole egg in about 30 seconds. Ca²⁺ is the second messenger for egg activation. Watch Embryology | Fertilization, Cleavage, Blastulation on YouTube, from Ninja Nerd
- Two blocks to polyspermy. Fast block = electrical: a brief change in egg membrane potential (needs Na⁺ influx) stops further sperm fusion. Slow block = physical: cortical granules release enzymes that strip sperm receptors and harden the coat into a fertilization envelope. Watch Cortical Reaction in Sea Urchin | Slow Blocks to Polyspermy on YouTube, from Hussain Biology
- Polyspermy makes a polyploid zygote, which is lethal in animals (but a speciation route in plants). About 10% of human spontaneous abortions are triploid, and ~70% of those come from dispermy. Watch Cortical Reaction in Sea Urchin | Slow Blocks to Polyspermy on YouTube, from Hussain Biology
- In frogs, sperm entry drives cortical rotation, exposing the gray crescent on the side opposite the entry point. The gray crescent marks the first cleavage plane and the future dorsal lip of the blastopore. Watch Spemann-Mangold Organizer and embryonic induction on YouTube, from Zar
- Cleavage is rapid division with no increase in embryo size — cells just get smaller. It is fast because the G1 and G2 gap phases are shortened or skipped. Watch Embryology | Fertilization, Cleavage, Blastulation on YouTube, from Ninja Nerd
- Yolk is the single biggest factor controlling cleavage pattern. Little or moderate yolk → holoblastic (furrow cuts all the way through). Lots of yolk → meroblastic (furrow cannot cut the yolk). Watch Stages of Animal Development: Cleavage, Gastrulation, Organogenesis on YouTube, from Professor Dave Explains
- Holoblastic types: radial (echinoderms, isolecithal), spiral (annelids, mollusks, flatworms), rotational (mammals, nematodes), displaced radial (amphibians, mesolecithal). Meroblastic types: discoidal (fish, reptiles, birds — telolecithal) and syncytial (most insects — centrolecithal).
- The blastocoel forms when outer blastomeres seal together with tight junctions, then interior cells pump Na⁺ out into the intercellular spaces and water follows osmotically. Watch Embryology | Fertilization, Cleavage, Blastulation on YouTube, from Ninja Nerd
- The mammalian blastula is a blastocyst: outer trophoblast (→ placenta) + inner cell mass (→ the embryo itself) + blastocoel. The ICM is the mammalian equivalent of the bird blastodisc. Watch Reproductive System, Part 4 - Pregnancy & Development: Crash Course Anatomy & Physiology #43 on YouTube, from CrashCourse
- Mammals show regulative development — early blastomeres are NOT committed, so removing one cell from an 8-cell embryo still gives a whole baby, and a split embryo gives identical twins. Most other animals are more mosaic (cytoplasmic determinants fix fate early).
- Compaction at the 8-cell stage flattens and polarizes blastomeres. Cells that end up inside become ICM; cells left outside become trophoblast.
- Gastrulation turns the hollow blastula into a three-layered, bilaterally symmetrical embryo with a primitive gut and defined anterior–posterior and dorsal–ventral axes. Watch Embryology | Gastrulation on YouTube, from Ninja Nerd
- Four gastrulation movements: invagination (sheet dents inward — low yolk), involution (sheet rolls inward over another — high yolk), ingression (single cells break away and migrate), delamination (one sheet splits into two — birds and mammals). Watch Embryology | Gastrulation on YouTube, from Ninja Nerd
- Sea urchin: vegetal plate → primary mesenchyme cells ingress (future mesoderm) → the rest invaginates as the archenteron → its opening, the blastopore, becomes the anus. Anus first = deuterostome. Watch Embryology | Gastrulation on YouTube, from Ninja Nerd
- Frog: cells involute over the dorsal lip of the blastopore (formed where the gray crescent was); a yolk plug fills the opening; the archenteron erases the blastocoel. Watch Spemann-Mangold Organizer and embryonic induction on YouTube, from Zar
- Birds and mammals: cells ingress through the primitive streak, which is functionally an elongated blastopore. First-through cells become endoderm, next become mesoderm, cells that never enter stay ectoderm. Watch Embryology | Gastrulation on YouTube, from Ninja Nerd
- The four extraembryonic membranes. Amnion — fluid-filled sac around the embryo, an artificial pond. Chorion — outermost; gas exchange under the shell in birds, the fetal half of the placenta in mammals. Yolk sac — nutrition in birds/reptiles, present but non-nutritive in mammals. Allantois — uric acid waste storage in birds, umbilical cord blood vessels in mammals. Watch Extraembryonic Membranes on YouTube, from Educator.com
- In birds the allantois fuses with the chorion to make the chorioallantoic membrane, the embryo's respiratory surface under the porous shell. Watch Extraembryonic Membranes on YouTube, from Educator.com
- Vertebrate organogenesis opens with the two chordate signatures: the notochord (mesoderm rod) and the hollow dorsal nerve cord. Building the nerve cord is neurulation: neural plate → neural groove → neural tube, driven by apical actin contraction wedging the cells. Watch Embryology | Neurulation, Vesiculation, Neural Crest Cell Migration on YouTube, from Ninja Nerd
- Neural crest cells pinch off the dorsal margin of the closing neural tube. They exist only in vertebrates and are a hallmark of vertebrate evolution. Ventral pathway → dorsal root ganglia, Schwann cells, autonomic ganglia, adrenal medulla. Lateral pathway → skin pigment cells. Watch Embryology of Nervous System - Neurulation - Neural Tube & Neural Crest - Embryonic Disc Folding on YouTube, from Medicosis Perfectionalis
- Mesoderm lineages: chorda-mesoderm → notochord; paraxial → head mesoderm and somites (cartilage, skeletal muscle, dermis); intermediate → kidneys and gonads; lateral plate → circulatory system, body-cavity linings, extraembryonic tissue. The coelom is the gap between the two split lateral layers. Watch Germ layer derivatives | Behavior | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Spemann–Mangold organizer = the dorsal lip of the blastopore. Transplanted to the belly of a second embryo it recruits host cells to build a second complete body axis. The amniote equivalent is Hensen's node. Watch Spemann-Mangold Organizer and embryonic induction on YouTube, from Zar
- The organizer works by inhibition, not instruction. All prospective mesoderm makes BMP4 (which pushes ventral fates); the organizer secretes BMP4 antagonists — Noggin, Chordin, Dickkopf, Cerberus — so nearby cells go dorsal by default. Blocking BMP4 in ectoderm makes neural tissue instead of epidermis. Watch Spemann-Mangold Organizer and embryonic induction on YouTube, from Zar
- Limb pattern formation, three control points: the AER secretes FGF and drives proximal–distal outgrowth; the ZPA secretes Shh and sets anterior–posterior digit identity (extra ZPA = mirror-image digits); the progress zone mesenchyme sets limb identity (forelimb vs hindlimb) via Hox genes. Tbx5 = forelimb, Tbx4 = hindlimb. Watch Apical Ectodermal Ridge, zone polarizing Activity Everything You Need To Know - Dr. Nabil Ebraheim on YouTube, from nabil ebraheim Watch Homeobox genes on YouTube, from Shomu's Biology
- Human gestation averages 266 days. First cleavage ~30 hours; blastocyst reaches the uterus at 6–7 days; implantation ends week 1; placenta + gastrulation in week 2; neurulation week 3; organogenesis and heartbeat week 4; embryo becomes a fetus at the 9th week. hCG from the trophoblast keeps the corpus luteum alive and is what pregnancy tests detect. Watch Reproductive System, Part 4 - Pregnancy & Development: Crash Course Anatomy & Physiology #43 on YouTube, from CrashCourse