Gastrulation in Frog: Stages & Blastopore
Quick answer: Gastrulation in the frog is the reorganization that turns the hollow blastula into the three-layered gastrula (ectoderm, mesoderm, endoderm) with a new cavity, the archenteron. Starting at the dorsal lip of the blastopore, bottle cells invaginate and movements (involution, epiboly, convergence) roll cells inside, displacing the blastocoel.
Key Facts: Gastrulation in Frog
- Gastrulation converts the hollow, single-layered blastula into the three-layered gastrula (ectoderm, mesoderm, endoderm). It is the biggest remodeling job in early development.
- It begins at the dorsal lip of the blastopore, in the marginal zone near the gray crescent region.
- Bottle cells (flask-shaped cells) start the groove; then involution, epiboly, convergence and divergence move the cells into place.
- The archenteron (primitive gut) forms and grows while the blastocoel is displaced and eventually obliterated.
- The dorsal lip of the blastopore is Spemann's organizer: transplant it, and it can induce a second embryonic axis.
- In the frog (a deuterostome), the blastopore becomes the anus; the mouth forms secondarily.
Cleavage is just the embryo getting chopped into smaller pieces. Gastrulation is where the real architecture begins. In the frog, a hollow ball of cells reorganizes itself into an embryo with three primary germ layers, ectoderm, mesoderm and endoderm, and a brand-new cavity, the archenteron (the primitive gut). Cells migrate, roll inward, and stretch across the surface in a choreography so precise that embryologists have studied it for a hundred years. Let's walk through it step by step, because "describe gastrulation in frog" is a guaranteed 5-mark question.
The Sequence: Morula → Blastula → Gastrula
Gastrulation is the third chapter of early development, and each chapter sets up the next:
- Morula: cleavage divides the zygote into a solid ball of cells with no cavity.
- Blastula: cells arrange into a hollow ball around a fluid-filled cavity, the blastocoel (segmentation cavity).
- Gastrula: gastrulation reorganizes the blastula into three germ layers (ectoderm, mesoderm, endoderm) around a new cavity, the archenteron, while the blastocoel disappears.
The Frog Blastula Before Gastrulation
After unequal holoblastic cleavage, the frog blastula shows clear polarity. Get this picture fixed in your mind first, because everything that follows depends on it:
- Animal hemisphere: small, numerous micromeres with little yolk; the blastocoel (segmentation cavity) lies here, displaced toward the animal pole.
- Vegetal hemisphere: large, yolk-laden macromeres that divide slowly.
- Marginal (equatorial) zone: the belt between the two hemispheres. Remember this belt well: gastrulation starts here.
- Gray crescent: a lightly pigmented band on the future dorsal side (opposite the sperm entry point), marking exactly where the dorsal blastopore lip will form.
How Gastrulation Begins: The Dorsal Lip of the Blastopore
Gastrulation starts at the marginal zone on the future dorsal side, just below the equator in the gray-crescent region. Here, marginal endodermal cells change shape and become bottle (flask-shaped) cells: their outer ends stay attached to the surface while their bodies pull inward, creating the first small groove, the beginning of the blastopore. The upper edge of this groove is the dorsal lip of the blastopore, and it is the site where the great cell migrations of gastrulation begin. (Look at their shape in the diagram: narrow neck at the surface, round body pulled inward. Like little bottles. The name sticks once you see it.)
Morphogenetic Movements of Frog Gastrulation
Now the movements. Learn these as a story, not a list: the bottle cells dig the ditch, then cells roll inward over the lip (involution), the top sheet stretches over the whole embryo (epiboly), cells stream toward the midline (convergence), and the embryo lengthens (divergence/extension).
| Movement | What happens |
|---|---|
| Invagination | The bottle cells buckle inward, starting the blastopore groove and the archenteron. |
| Involution | Marginal-zone cells roll inward over the blastopore lip and migrate along the inner surface of the blastocoel roof, becoming mesoderm and endoderm. The cells forming the lip are constantly changing as new cells arrive. |
| Epiboly | The animal-cap ectoderm cells spread and thin out, expanding over the whole embryo and eventually covering the yolk: the blastopore lip appears to shrink and move ventrally. |
| Convergence | Cells from the lateral and ventral regions move toward the dorsal midline, streaming toward the blastopore. |
| Divergence (extension) | Cells spread out and extend along the anterior-posterior axis, lengthening the embryo. |
Fate of the Moving Cells: The Three Germ Layers
Where does every migrating cell end up? This is the part examiners probe most:
- The first cells to involute (endodermal) form the leading edge of the archenteron and later become pharyngeal (foregut) endoderm.
- Next, head mesoderm and then chordamesoderm involute: the latter forms the notochord, the transient mesodermal "backbone" that induces the nervous system.
- Cells entering through the lateral and ventral lips form the mesodermal mantle (somites, kidneys, heart, blood).
- The cells remaining on the surface become ectoderm (future skin and nervous system); the involuted sheet becomes mesoderm; the archenteron roof and floor become endoderm (future gut lining).
As the archenteron grows, the blastocoel shrinks and disappears; the blastopore narrows to a small opening temporarily plugged by yolk cells (the yolk plug stage). The finished gastrula is a three-layered embryo with the archenteron as its only cavity.
Derivatives of the Three Germ Layers
The germ layers are defined by what they become. Learn this nested map; it underlies every organ-system question in embryology:
- Ectoderm (outer layer; stays on the surface)
- Nervous system: brain, spinal cord, peripheral nerves (from the neural tube and neural crest)
- Epidermis: skin, hair, nails, sweat glands
- Sense organs: lens of the eye, inner ear
- Mesoderm (middle layer; involutes through the blastopore lips)
- Muscle: skeletal, cardiac and smooth muscle (from somites)
- Skeleton: bones and cartilage
- Circulatory system: heart, blood vessels, blood
- Excretory system: kidneys and gonads
- Notochord: the transient organizer of the nervous system (from chordamesoderm)
- Endoderm (inner layer; lines the archenteron)
- Gut lining: epithelial lining of the digestive tract
- Respiratory lining: lining of the lungs
- Digestive glands: liver, pancreas
For "describe gastrulation in frog" (a standard 5-mark BS question), structure your answer exactly like this: (1) blastula structure (animal/vegetal poles, blastocoel), (2) initiation: dorsal lip of the blastopore, bottle cells, gray crescent, (3) the four movements (involution, invagination, epiboly, convergence/divergence), (4) result: archenteron replaces blastocoel, three germ layers formed. Writing "gastrulation begins at the animal pole" loses the mark. Examiners specifically check the phrase "begins at the dorsal lip of the blastopore". And at BS level, always name the dorsal lip as Spemann's organizer.
Spemann's Organizer: Why the Dorsal Lip Is Famous
In the 1920s, Hans Spemann and Hilde Mangold did one of the most elegant experiments in biology: they transplanted the dorsal lip of the blastopore from one newt gastrula onto another embryo, and a second, complete body axis (including a second nervous system) developed. The dorsal lip is therefore called Spemann's organizer (the primary organizer): it not only forms dorsal structures itself but instructs neighboring cells (by secreting signaling molecules) to become neural tissue. This single experiment founded modern experimental embryology.
After gastrulation, development continues with neurulation (formation of the neural tube) and organogenesis. In amniotes, the embryo then builds the extraembryonic membranes (amnion, chorion, allantois, yolk sac) that protect, nourish and aerate it. The frog, as an anamniote, skips these membranes entirely and develops in water.
Frequently Asked Questions
Where exactly does gastrulation begin? I always get confused.
At the dorsal lip of the blastopore, in the marginal zone on the future dorsal side of the embryo (the gray-crescent region). This is the single most-asked exam fact on frog gastrulation, so fix it in memory.
What are bottle cells? The name sounds funny.
Bottle (flask-shaped) cells are the first cells at the dorsal blastopore lip to change shape during gastrulation: their outer ends stay attached to the surface while their bodies move inward, creating the initial groove of the blastopore.
What is this archenteron everyone keeps mentioning?
The archenteron (primitive gut) is the new cavity formed during gastrulation as cells involute through the blastopore. It grows while the blastocoel shrinks, and its lining becomes the endoderm of the future gut.
Blastula vs gastrula, what is the difference?
The blastula is the hollow, single-layered ball of cells resulting from cleavage (with a blastocoel). The gastrula is the three-layered embryo (ectoderm, mesoderm, endoderm) produced by gastrulation, with the archenteron as its cavity.
Why is Spemann's organizer such a big deal?
The dorsal lip of the blastopore in amphibian gastrulae is called Spemann's organizer because, when transplanted, it induces a second embryonic axis and neural development in the host, proving it organizes the body plan. It is the classic example of embryonic induction.
What happens to the blastopore after gastrulation?
In the frog, a deuterostome, the blastopore becomes the anus; the mouth forms secondarily at the opposite end. In protostomes (most invertebrates), the blastopore becomes the mouth instead. This is the defining difference between the two great animal lineages.
Practice MCQs
Q1. In amphibians, gastrulation begins at the:
- Animal pole
- Vegetal pole
- Dorsal lip of the blastopore
- Blastocoel roof
Answer: C — Involution starts at the dorsal blastopore lip in the gray-crescent region.
Q2. The primitive gut formed during gastrulation is called the:
- Blastocoel
- Archenteron
- Blastopore
- Neurenteron
Answer: B — The archenteron grows as the blastocoel is displaced and obliterated.
Q3. The inward rolling of marginal-zone cells over the blastopore lip is called:
- Epiboly
- Involution
- Invagination
- Delamination
Answer: B — Involution is the turning-inward of cells over the lip; epiboly is the spreading of the surface ectoderm.
Q4. Spemann's organizer is the:
- Ventral lip of the blastopore
- Dorsal lip of the blastopore
- Animal pole
- Yolk plug
Answer: B — The dorsal lip induces a second body axis when transplanted: the primary organizer.
Q5. Which germ layer lines the archenteron?
- Ectoderm
- Mesoderm
- Endoderm
- Mesoglea
Answer: C — The archenteron is lined by endoderm, which forms the gut lining and associated organs.
Q6. Bottle cells are:
- The yolk-laden macromeres of the vegetal pole
- The endodermal cells forming the archenteron roof
- The thin cells of the blastocoel roof
- Flask-shaped marginal cells whose inward pull starts the blastopore groove
Answer: D — Marginal endodermal cells become flask-shaped bottle cells; their outer ends stay attached to the surface while their bodies pull inward, creating the first groove of the blastopore.
Q7. As the archenteron grows during frog gastrulation, the blastocoel:
- Shrinks and is eventually obliterated
- Enlarges to become the primitive gut
- Merges with the archenteron
- Shifts permanently to the vegetal pole
Answer: A — The archenteron grows as the blastocoel is displaced and disappears, so the finished gastrula has the archenteron as its only cavity.
Q8. The gray crescent region of the frog blastula is significant because it:
- Marks the future ventral side of the embryo
- Shows exactly where the sperm entered
- Contains the largest concentration of yolk
- Marks the future dorsal side, where the dorsal blastopore lip will form
Answer: D — The gray crescent is a lightly pigmented band on the future dorsal side (opposite the sperm entry point), marking where the dorsal blastopore lip forms.
Q9. The notochord of the frog embryo forms from:
- Chordamesoderm, which involutes after the head mesoderm
- The animal-cap ectoderm
- Endodermal cells of the archenteron floor
- The cells of the yolk plug
Answer: A — After the head mesoderm, chordamesoderm involutes and forms the notochord, the transient mesodermal backbone that induces the nervous system.
Q10. Which of the following is NOT formed from mesoderm during frog development?
- Somites
- The gut lining
- Heart
- Kidneys
Answer: B — Somites, heart, kidneys and blood come from the mesodermal mantle; the gut lining (archenteron roof and floor) becomes endoderm.
Q11. The yolk plug stage of frog gastrulation is seen when:
- The blastopore has narrowed to a small opening temporarily plugged by yolk cells
- The archenteron is fully complete
- Neurulation begins
- The blastocoel reforms
Answer: A — As the archenteron grows, the blastopore narrows to a small opening plugged by yolk cells: the yolk plug stage.
Q12. In deuterostomes (including the frog), the blastopore becomes the:
- Mouth
- Anus
- Neural tube
- Notochord
Answer: B — In deuterostomes the blastopore becomes the anus and the mouth forms secondarily; in protostomes the blastopore becomes the mouth.
Short Questions
Q1. What is gastrulation, and what does it produce? See answer
Gastrulation is the phase of embryonic development in which the hollow, single-layered blastula is reorganized into the gastrula: a three-layered embryo with the primary germ layers ectoderm (outer), mesoderm (middle) and endoderm (inner), surrounding a new cavity, the archenteron (primitive gut).
It follows cleavage, which merely divides the zygote into many small cells without changing the total volume. Where cleavage produces quantity, gastrulation produces architecture: the germ layers are the raw material from which every organ system will later be built.
The finished gastrula has the archenteron as its only cavity, because the original blastocoel is displaced and obliterated as the archenteron grows. In the frog the blastopore, the opening of the archenteron, persists as the future anus.
Q2. Where does gastrulation begin in the frog, and how? See answer
Gastrulation begins at the dorsal lip of the blastopore, in the marginal zone on the future dorsal side of the embryo, in the gray-crescent region (opposite the sperm entry point). This single fact is the most-asked exam point on frog gastrulation.
It starts when marginal endodermal cells change shape into bottle (flask-shaped) cells: their outer ends stay attached to the surface while their bodies pull inward, buckling the surface into the first small groove, the beginning of the blastopore. The upper edge of this groove is the dorsal lip.
The dorsal lip is not just a starting point; it is Spemann's organizer, the primary organizer that instructs neighboring cells to become neural tissue. Never write that gastrulation begins at the animal pole: examiners specifically check for the dorsal lip.
Q3. Distinguish between involution and epiboly. See answer
Involution is the inward rolling of marginal-zone cells over the blastopore lip: cells turn in at the lip and migrate along the inner surface of the blastocoel roof, becoming the future mesoderm and endoderm. The cells forming the lip are constantly changing as new cells arrive to replace them.
Epiboly is the spreading and thinning out of the animal-cap ectoderm cells, which expand to cover the whole embryo and eventually engulf the yolk-laden vegetal cells. As epiboly proceeds, the blastopore lip appears to shrink and move toward the ventral side.
The two movements cooperate: while involution carries cells inside, epiboly stretches the surface sheet over them, so the outside of the finished gastrula is pure ectoderm and the inside holds the mesoderm and endoderm. A third movement, convergence, streams lateral and ventral cells toward the dorsal midline to feed the blastopore.
Q4. What is the archenteron, and what happens to the blastocoel during gastrulation? See answer
The archenteron (primitive gut) is the new cavity formed during gastrulation as cells involute through the blastopore. It grows steadily inward and forward; its roof and floor are lined by endoderm, which will form the gut lining, liver, pancreas and lungs.
As the archenteron grows, the original blastocoel (segmentation cavity) is displaced and eventually obliterated: it shrinks until it disappears entirely. The finished gastrula therefore has the archenteron as its only cavity.
The blastopore, the opening through which the archenteron communicates with the outside, narrows to a small opening temporarily plugged by yolk cells (the yolk plug stage). In the frog, a deuterostome, this opening becomes the anus.
Q5. What is Spemann's organizer, and why is it famous? See answer
Spemann's organizer is the dorsal lip of the blastopore in amphibian gastrulae. In the 1920s, Hans Spemann and Hilde Mangold transplanted this lip from one newt gastrula onto another embryo, and a second, complete body axis (including a second nervous system) developed.
The experiment proved that the dorsal lip does not merely form dorsal structures itself; it instructs neighboring cells (by secreting signaling molecules) to become neural tissue. This phenomenon, called embryonic induction, founded modern experimental embryology and earned Spemann the Nobel Prize.
For exams, remember it as the primary organizer and the classic example of induction. Any question about "the organizer" in amphibian development expects the answer: the dorsal lip of the blastopore.
Q6. Which germ layer forms the notochord, and what is its role? See answer
The notochord forms from chordamesoderm, a dorsal mesoderm that involutes through the dorsal blastopore lip after the head mesoderm. It is the transient mesodermal "backbone" lying beneath the future nervous system.
Its role is inductive, not structural: the notochord secretes signals that induce the overlying ectoderm to become the neural plate, the first step of nervous-system formation. Without the notochord, there is no brain and no spinal cord.
Other mesodermal fates include somites (muscle and skeleton), heart and blood, and kidneys; the surface cells that never involute become ectoderm (skin, nervous system), while the archenteron lining becomes endoderm (gut lining, liver, pancreas, lungs).
References
- Hickman, C.P., Roberts, L.S. & Larson, A. Integrated Principles of Zoology, 18th ed. McGraw-Hill. (Gastrulation; amphibian development.)
- Miller, S.A. & Harley, J.P. Zoology, McGraw-Hill. (Morphogenetic movements; Spemann organizer.)
- OpenStax, Biology 2e, Chapter 43: Animal Reproduction and Development: https://openstax.org/details/books/biology-2e.
- NCBI Bookshelf — Developmental Biology: Gastrulation in amphibians: https://www.ncbi.nlm.nih.gov/books/.
Comments
Leave a comment