The three germ layers — the single most tested item in the chapter
3 rows| Germ layer | Adult structures | Memory hook |
|---|---|---|
| Ectoderm | Epidermis of the skin, hair, lens of the eye, inner ear; nervous system (brain, spinal cord, spinal nerves) via the neural tube; sense organs; neural crest → gill arches, sensory ganglia, Schwann cells, adrenal medulla, pigment cells | Everything you touch the world with: outer covering + wiring |
| Mesoderm | Notochord; skeleton and cartilage; skeletal and striated muscle; dermis of the skin; heart, blood, blood vessels; kidneys; gonads; linings of the thoracic and abdominal cavities | Everything in between: bone, muscle, blood, plumbing |
| Endoderm | Lining of the digestive tract; lining of the respiratory tract and pharynx; liver; pancreas; thymus; thyroid | The tube through you, plus the glands that bud off it |
Yolk decides the cleavage pattern
6 rows| Yolk type | Cleavage | Geometry | Examples |
|---|---|---|---|
| Isolecithal (sparse, even) | Holoblastic | Radial — cells in neat stacked rows | Echinoderms |
| Isolecithal | Holoblastic | Spiral — daughters angled over cells below | Annelids, mollusks, flatworms |
| Isolecithal | Holoblastic | Rotational — 2nd division meridional in one cell, equatorial in the other | Mammals, nematodes |
| Mesolecithal (moderate, vegetal) | Holoblastic | Displaced radial — furrows shifted toward the animal pole | Amphibians |
| Telolecithal (dense throughout) | Meroblastic | Discoidal — division confined to a blastodisc on the yolk | Fish, reptiles, birds |
| Centrolecithal (central) | Meroblastic | Syncytial — nuclei divide with no membranes at first | Most insects |
The four extraembryonic membranes
4 rows| Membrane | Job in birds and reptiles | Job in mammals |
|---|---|---|
| Amnion | Inner sac holding amniotic fluid; suspends the embryo in a private pond | Same — the fluid-filled sac around the fetus |
| Chorion | Outermost membrane against the shell; separated from the rest by the extraembryonic coelom | From the trophoblast; its contact zone becomes the fetal half of the placenta |
| Yolk sac | The embryo food supply — critical for nutrition | Still formed, but non-nutritive; nutrients come from the placenta |
| Allantois | Stores uric acid waste; fuses with chorion as the chorioallantoic membrane for gas exchange | Contributes the blood vessels of the umbilical cord |
Gastrulation across the groups
4 rows| Group | Yolk / blastula | Main movement | Opening |
|---|---|---|---|
| Sea urchin | Low yolk, symmetric hollow blastula | Ingression of PMCs, then invagination of the vegetal plate | Blastopore → anus (deuterostome) |
| Frog | Mesolecithal, eccentric blastocoel | Involution over the dorsal lip | Blastopore with a yolk plug |
| Bird / reptile | Telolecithal, blastoderm on huge yolk | Delamination then ingression | Primitive streak |
| Mammal | Almost no yolk, blastocyst with ICM | Delamination of the ICM, then ingression | Primitive streak — copies the bird |
Limb bud control centers
3 rows| Region | Signal | What it controls | Key experiment |
|---|---|---|---|
| AER (apical ectodermal ridge) | FGF | Proximal–distal outgrowth; NOT limb identity | Remove early = truncated limb; remove late = only distal parts missing; add a second AER = two limbs |
| ZPA (zone of polarizing activity) | Sonic hedgehog (Shh) | Anterior–posterior digit identity — high Shh = posterior digits | Graft a second ZPA = mirror-image duplicate digits |
| Progress zone (mesenchyme under the AER) | Hox gene activity | Limb identity: forelimb (Tbx5) vs hindlimb (Tbx4) | Forelimb AER grafted to a hindlimb site still makes a hindlimb |
Exam traps
11 pairsQuestions get built out of near-misses. If you can state each difference in one sentence, you will not lose those points.
BlastulavsGastrula
Blastula = hollow ball, one cell layer around a blastocoel, made by cleavage. Gastrula = three germ layers plus a primitive gut (archenteron), made by cell movement. Blastula has a blastocoel; gastrula has an archenteron.
Ectoderm derivativesvsEndoderm derivatives
Ectoderm = the outside and the nervous system (epidermis, brain, spinal cord, sense organs). Endoderm = the linings of the tubes inside (gut lining, respiratory lining) plus liver, pancreas, thymus, thyroid. If it is a gland budding off the gut, it is endoderm.
Epidermis of the skinvsDermis of the skin
Skin is a two-layer trick question. Epidermis = ectoderm. Dermis = mesoderm, from the somites. Do not answer "ectoderm" for the whole skin.
AmnionvsChorion
Amnion is the inner sac holding the fluid the embryo floats in. Chorion is the outermost membrane — against the shell in birds, the fetal side of the placenta in mammals.
AllantoisvsYolk sac
Allantois handles waste and vessels: uric acid storage in birds, umbilical cord blood vessels in mammals. Yolk sac handles food: nutrition in birds and reptiles, present but non-nutritive in mammals.
Mosaic developmentvsRegulative development
Mosaic = fate fixed early by cytoplasmic determinants; kill a cell and its structures are permanently missing (tunicates). Regulative = fate not fixed; remove a cell and the rest compensate (mammals — this is why identical twins exist).
Holoblastic cleavagevsMeroblastic cleavage
Holoblastic = the furrow cuts all the way through (little or moderate yolk: echinoderms, frogs, mammals). Meroblastic = the furrow cannot cut the yolk (heavy yolk: fish, reptiles, birds, insects). Mammals are holoblastic even though their development otherwise mimics birds.
Fast block to polyspermyvsSlow block to polyspermy
Fast = electrical, a brief membrane potential change requiring Na⁺ influx, immediate. Slow = physical/chemical, cortical granules strip sperm receptors and harden the coat into the fertilization envelope.
BlastoporevsPrimitive streak
Both are the ingression gateway, but blastopore is a hole (sea urchins, frogs) and the primitive streak is an elongated furrow (birds, mammals). Functionally the streak is a stretched-out blastopore.
AERvsZPA
AER sits at the tip of the bud, secretes FGF, controls how far the limb grows out (proximal–distal). ZPA is mesenchyme, secretes Shh, controls which digit is which (anterior–posterior). Neither one sets forelimb vs hindlimb — that is the progress zone.
Primary inductionvsSecondary induction
Primary = between the three germ layers (dorsal ectoderm + dorsal mesoderm → neural tube). Secondary = between tissues already committed (optic stalk → lens). If the example is the eye lens, the answer is secondary.