Gastrulation: The Fundamental Stages of Early Embryonic Development

 Cleavage and gastrulation are not isolated events but represent a tightly integrated continuum of early embryonic development. Cleavage, by rapidly increasing cell number and establishing a proper nuclear-to-cytoplasmic ratio, provides the essential cellular raw material and initial organizational framework necessary for gastrulation. Gastrulation, in turn, takes these newly formed cells and orchestrates their precise movements and rearrangements to form the fundamental three-layered body plan. This process establishes the primary germ layers, which are the foundational blueprints for all future tissues and organs, and thereby meticulously sets the stage for the intricate process of organogenesis. The successful execution of all subsequent developmental stages, where specific organs and tissues differentiate and mature, is entirely contingent upon the accurate and coordinated completion of these foundational processes. Understanding their intricate mechanisms and the fascinating variations observed across different species provides profound insights into the remarkable precision, inherent adaptability, and deep evolutionary history embedded within life's earliest moments.

 Gastrulation: Shaping the Embryo

Gastrulation is a crucial and profoundly dynamic stage in early embryonic development where the simple, hollow blastula undergoes extensive and coordinated cell movements and rearrangements. This intricate process transforms the blastula into a multi-layered structure known as the gastrula (Bayquen, n.d.; CK-12 Foundation, n.d.; Gilbert, 2000).

The fundamental objective of gastrulation is to establish the primary germ layers—the ectoderm, mesoderm, and endoderm—and to lay out the basic body plan of the organism (CK-12 Foundation, n.d.; Fiveable, n.d.). This process precisely positions embryonic cells in their correct locations relative to one another, thereby setting the essential foundation for all future tissue and organ formation, a subsequent phase known as organogenesis (CK-12 Foundation, n.d.; Fiveable, n.d.).

Formation of Primary Germ Layers

 

Gastrulation is widely considered a pivotal evolutionary innovation in the animal kingdom (Riesgo & Wessel, 2021). This process provides the basic embryonic architecture, fundamentally creating an inner layer distinct from an outer layer, from which all diverse animal forms have subsequently arisen (Riesgo & Wessel, 2021). This highlights that gastrulation was not merely an incremental developmental step but a transformative evolutionary leap. Before its emergence, organisms were likely simpler, perhaps resembling sac-like structures. The formation of distinct inner and outer layers, followed by a middle layer, enabled the specialization of tissues and the development of complex internal organs. This innovation provided the fundamental blueprint for bilateral symmetry and facilitated the subsequent diversification of body plans observed across the vast array of animal life. Consequently, comprehending gastrulation extends beyond mere embryology; it provides a profound appreciation for a critical moment in the history of life, enabling the emergence of complex multicellular animals from simpler ancestral forms. It represents the point at which multicellularity acquired true internal organization and complexity.

Formation of the Primary Germ Layers

The primary germ layers are three fundamental embryonic cell layers—the ectoderm, mesoderm, and endoderm—that are meticulously established during gastrulation (CK-12 Foundation, n.d.; Fiveable, n.d.). Each of these layers is precisely fated to give rise to specific tissues, organs, and entire organ systems within the developing organism (CK-12 Foundation, n.d.; Fiveable, n.d.; OpenStax, n.d.).

• Ectoderm: This constitutes the outermost germ layer of the gastrula (CK-12 Foundation, n.d.; Fiveable, n.d.). It is responsible for giving rise to the epidermis, which includes the skin and its various appendages such as hair and nails (Fiveable, n.d.). Crucially, the ectoderm also forms the entire nervous system, encompassing the brain, spinal cord, and peripheral nerves, as well as sensory organs like the eyes, ears, and nose (Fiveable, n.d.; OpenStax, n.d.). A specialized group of cells originating from the ectoderm, known as neural crest derivatives, further diversifies to form structures including facial cartilage, bones, melanocytes (pigment-producing cells), and components of the adrenal medulla (Fiveable, n.d.).
• Mesoderm: Positioned as the middle germ layer, the mesoderm is situated between the ectoderm and the endoderm (CK-12 Foundation, n.d.; Fiveable, n.d.). This versatile layer develops into the musculoskeletal system, which comprises muscles, bones, and cartilage (Fiveable, n.d.; OpenStax, n.d.). It also forms the circulatory system, including the heart, blood vessels, and blood cells, and the urogenital system, which includes the kidneys and gonads (Fiveable, n.d.). Furthermore, the mesoderm contributes to various connective tissues and the dermis of the skin (Fiveable, n.d.; OpenStax, n.d.). Key axial structures such as the notochord and somites also originate from the mesoderm (Fiveable, n.d.).
• Endoderm: The endoderm represents the innermost germ layer of the gastrula (CK-12 Foundation, n.d.; Fiveable, n.d.). It forms the epithelial lining of the entire digestive tract, including the gut, and the respiratory system (Fiveable, n.d.; OpenStax, n.d.). Associated organs such as the liver, pancreas, thyroid, thymus, and parathyroid glands also develop from the endoderm (Fiveable, n.d.). Additionally, it contributes to the epithelial lining of the bladder and urethra (Fiveable, n.d.).

The table below summarizes the major tissues and organs derived from each primary germ layer:

Germ Layer	Major Tissues and Organs Derived
Ectoderm	Epidermis (skin, hair, nails), Nervous system (brain, spinal cord, peripheral nerves), Sensory organs (eyes, ears, nose), Epithelial lining of mouth and anus, Neural crest derivatives (facial cartilage, bones, melanocytes, adrenal medulla) (Fiveable, n.d.; OpenStax, n.d.)
Mesoderm	Musculoskeletal system (muscles, bones, cartilage), Circulatory system (heart, blood vessels, blood), Urogenital system (kidneys, gonads), Connective tissues, Dermis of the skin, Notochord, Somites (Fiveable, n.d.; OpenStax, n.d.)
Endoderm	Epithelial lining of gastrointestinal tract and respiratory system, Liver, Pancreas, Thyroid, Thymus, Parathyroid glands, Epithelial lining of bladder and urethra, Digestive glands (Fiveable, n.d.; OpenStax, n.d.)

Derivatives of the Primary Germ Layers

Major Morphogenetic Cell Movements

Morphogenetic movements are the highly coordinated cellular rearrangements and changes in cell shape that are the driving forces behind the complex transformations observed during gastrulation (Bayquen, n.d.; CK-12 Foundation, n.d.; Gilbert, 2000; SS CASC, n.d.). These movements are critical for precisely positioning cells to form the germ layers and to establish the fundamental body plan.

• Invagination: This movement involves an inward folding or inpushing of a sheet of cells, which creates a new cavity or an indentation within the embryonic structure (CK-12 Foundation, n.d.; Fiveable, n.d.; SS CASC, n.d.). It is often initiated by specific changes in cell shape, such as the apical constriction of bottle cells (Slideshare, n.d.; "Ingression (biology)", n.d.).
• Examples: The formation of the blastopore in amphibians (Gilbert, 2000; Slideshare, n.d.; SS CASC, n.d.) and the creation of the archenteron, or primitive gut, in sea urchin embryos (CK-12 Foundation, n.d.; Fiveable, n.d.; Riesgo & Wessel, 2021).

Morphogenetic Movement Invagination

• Involution:
• Involution: Involution describes the inward rolling or turning of an expanding outer layer of cells over the basal surface of an adjacent outer layer (CK-12 Foundation, n.d.; Fiveable, n.d.; SS CASC, n.d.). During this process, cells actively migrate along the inner surface of the outer layer.
• Examples: This movement is prominently observed as marginal zone cells roll over the blastopore lip and migrate into the interior of the amphibian embryo (Gilbert, 2000; Slideshare, n.d.; SS CASC, n.d.).

Morphogenetic Movement Involution

• Ingression:
• Ingression: Ingression refers to the migration of individual cells from a surface layer into the interior of the embryo (CK-12 Foundation, n.d.; Fiveable, n.d.; "Ingression (biology)", n.d.). This process frequently involves an epithelial-to-mesenchymal transition (EMT), where cells lose their typical epithelial characteristics, such as strong cell-to-cell adhesion, and acquire migratory properties (Fiveable, n.d.; "Ingression (biology)", n.d.). In sea urchins, primary mesenchyme cells (PMCs) undergo EMT, detaching from the epithelium, losing adhesion to the hyaline layer and cadherins, gaining affinity for the basal lamina, and reorganizing their cytoskeleton to become migratory ("Ingression (biology)", n.d.).
• Examples: Ingression occurs at the primitive streak in amniote embryos (birds and mammals), where epiblast cells migrate inward to form the mesoderm and endoderm (Fiveable, n.d.; Gilbert, 2000; Ichikawa et al., 2013; Synopsis IAS, 2024). It is also observed in the ingression of primary mesenchyme cells in sea urchins (Riesgo & Wessel, 2021; "Ingression (biology)", n.d.).

Morphogenetic Movement Ingression

• Delamination:
• Delamination: This movement involves the splitting of one cellular sheet into two distinct layers (CK-12 Foundation, n.d.; Fiveable, n.d.).
• Examples: The formation of the hypoblast in avian embryos (Fiveable, n.d.) and the separation of the epiblast from the primitive endoderm in mammalian blastocysts (Fiveable, n.d.; Gilbert, 2000).

Delamination, the splitting of one cellular sheet into two distinct layers

• Epiboly:
• Epiboly: Epiboly describes the thinning and spreading of a cell sheet over the surface of the embryo, effectively increasing its surface area to enclose deeper structures (CK-12 Foundation, n.d.; Fiveable, n.d.; SS CASC, n.d.).
• Examples: This movement is prominent in teleost fish gastrulation (Fiveable, n.d.) and is observed in amphibians as animal pole cells spread over the vegetal pole (Slideshare, n.d.; SS CASC, n.d.).

Epiboly, the thinning and spreading of a cell sheet over the surface of the embryo

• Convergent Extension:
• Convergent Extension: This is a morphogenetic movement where a tissue simultaneously narrows (converges) in one dimension while lengthening (extends) in a perpendicular dimension (Fiveable, n.d.; Riesgo & Wessel, 2021; SS CASC, n.d.). This is achieved through coordinated cell intercalation, where cells actively rearrange themselves into fewer, longer rows (Fiveable, n.d.).
• Examples: Convergent extension is a key driver of the elongation of the body axis in vertebrate embryos (Fiveable, n.d.). In sea urchins, it aligns cells into single files (Riesgo & Wessel, 2021). In zebrafish and frogs, cells approach from a 90-degree angle and randomly merge to extend the tissue (Riesgo & Wessel, 2021).

Convergent Extension

The intricate and coordinated nature of these cell movements is not random but is precisely controlled by underlying molecular cues. For instance, the process of ingression, particularly the epithelial-to-mesenchymal transition (EMT), is regulated by specific transcription factors like snail and twist, and signaling molecules such as beta-catenin ("Ingression (biology)", n.d.). Similarly, broader gastrulation movements are orchestrated by complex signaling pathways, including Wnt, Nodal, Fibroblast Growth Factor (FGF), and Bone Morphogenetic Protein (BMP) pathways (Fiveable, n.d.). FGF4 and FGF8 are specifically mentioned in chick gastrulation (Yang et al., 2002), and Wnt and BMP in chick gastrulation (Synopsis IAS, 2024). This highlights that these complex cellular behaviors are not merely descriptive events but are the result of a highly integrated genetic and biochemical regulatory network. This sophisticated communication system between cells dictates their behavior, fate, and ultimate position within the developing embryo. Understanding these molecular underpinnings is crucial for developmental biology research, as disruptions in these pathways can lead to congenital defects. Furthermore, it opens avenues for understanding pathological processes, such as cancer metastasis, which often involve EMT-like cellular transitions (Fiveable, n.d.).

 Comparative labelled image on Invagination, Involution and Ingression Morphogenetic Movement

The table below summarizes the major morphogenetic cell movements during gastrulation:

Movement Type	Description	Key Cellular Changes	Representative Examples/Organisms
Invagination	Inward folding/inpushing of a cell sheet, creating a new cavity.	Cell shape changes (e.g., apical constriction of bottle cells).	Amphibian blastopore formation (Gilbert, 2000; Slideshare, n.d.; SS CASC, n.d.), Sea urchin archenteron formation (CK-12 Foundation, n.d.; Fiveable, n.d.; Riesgo & Wessel, 2021).
Involution	Inward rolling of an expanding outer cell layer over an adjacent inner surface.	Cells migrate along inner surface.	Amphibian marginal zone cells rolling over blastopore lip (Gilbert, 2000; Slideshare, n.d.; SS CASC, n.d.).
Ingression	Migration of individual cells from a surface layer into the interior.	Epithelial-to-mesenchymal transition (EMT); loss of adhesion, gain of motility.	Primitive streak in amniotes (birds, mammals) (Fiveable, n.d.; Gilbert, 2000; Ichikawa et al., 2013; Synopsis IAS, 2024), Primary mesenchyme cells in sea urchins (Riesgo & Wessel, 2021; "Ingression (biology)", n.d.).
Delamination	Splitting of one cellular sheet into two distinct layers.	Separation of cell layers.	Hypoblast formation in avian embryos (Fiveable, n.d.), Epiblast/primitive endoderm separation in mammalian blastocysts (Fiveable, n.d.; Gilbert, 2000).
Epiboly	Thinning and spreading of a cell sheet over the embryo surface to enclose deeper structures.	Radial intercalation, cell expansion.	Teleost fish gastrulation (Fiveable, n.d.), Amphibian animal pole cells spreading over vegetal pole (Slideshare, n.d.; SS CASC, n.d.).
Convergent Extension	Tissue narrows in one dimension while lengthening in a perpendicular dimension.	Coordinated cell intercalation, polarized cell behaviors.	Elongation of body axis in vertebrates (Fiveable, n.d.), Archenteron extension in sea urchins (Riesgo & Wessel, 2021).

Species-Specific Gastrulation Patterns

While the fundamental goals of gastrulation—the formation of germ layers and the establishment of the basic body plan—are conserved across the animal kingdom, the precise cellular mechanisms, timing, and patterns of morphogenetic movements exhibit significant variations among different species (CK-12 Foundation, n.d.; Fiveable, n.d.). These variations often reflect evolutionary adaptations to diverse egg types, particularly differences in yolk content, and distinct life histories.

• Gastrulation in Sea Urchins: Sea urchin embryos serve as a remarkably simple and elegant model system for studying gastrulation (Riesgo & Wessel, 2021).
• Key Features: The process begins with the specification of cells early during cleavage (Riesgo & Wessel, 2021).
• Cell Movements:
• Primary Mesenchyme Cell (PMC) Ingression: The initial sign of gastrulation involves an epithelial-mesenchymal transition (EMT) of skeletogenic cells, known as primary mesenchyme cells (PMCs), from the vegetal plate. These cells then ingress into the blastocoel (Riesgo & Wessel, 2021; "Ingression (biology)", n.d.). During this process, PMCs lose their adhesion to the hyaline layer and cadherins, acquire an affinity for the basal lamina, and undergo a significant reorganization of their cytoskeleton ("Ingression (biology)", n.d.).
• Invagination of Vegetal Plate: Following the ingression of PMCs, the vegetal plate undergoes invagination, folding inward to form the archenteron, or primitive gut (Riesgo & Wessel, 2021). This invagination is partly driven by changes in cell shape within the vegetal plate (Riesgo & Wessel, 2021).
• Archenteron Elongation: The archenteron then elongates progressively towards the animal pole. This elongation is facilitated by filopodial extensions from secondary mesenchyme cells located at the archenteron's tip, which exert pulling forces on the blastocoel wall (Riesgo & Wessel, 2021). Convergent extension also contributes to the lengthening of the archenteron (Riesgo & Wessel, 2021).
• Resulting Structures: The primary outcomes include the formation of the archenteron, which will develop into the future gut, and the precise positioning of mesodermal cells within the embryo (Riesgo & Wessel, 2021).
• Gastrulation in Frogs (Amphibians):
• Gastrulation in Frogs (Amphibians): Gastrulation in amphibian embryos, such as frogs, is significantly influenced by the large amount of yolk present in their mesolecithal eggs, which leads to unequal cleavage and distinct animal and vegetal poles (Bastiani, n.d.). The process typically commences at the gray crescent, a region located opposite the point of sperm entry (Slideshare, n.d.).
• Key Features: The transformation of the single-layered blastula into a three-layered gastrula occurs through a series of dynamic morphogenetic movements (Slideshare, n.d.; SS CASC, n.d.).
• Cell Movements:
• Invagination (Blastopore Formation): Gastrulation is initiated by the invagination of bottle-shaped cells at the equatorial marginal zone, forming a slit-like blastopore (Slideshare, n.d.; SS CASC, n.d.). This dorsal blastopore lip is the first visible indication of gastrulation. The invagination creates a new cavity, the archenteron or gastrocoel (Slideshare, n.d.; SS CASC, n.d.).
• Involution: Marginal zone cells, which are presumptive mesoderm and endoderm, undergo involution, rolling inward over the blastopore lips (dorsal, lateral, and ventral) and migrating along the inner surface of the animal pole cells (Slideshare, n.d.; SS CASC, n.d.). The cells comprising the blastopore lip are constantly changing as new cells involute (Slideshare, n.d.).
• Epiboly: The micromeres, or smaller cells, of the animal pole undergo epiboly, a process of spreading and thinning to cover the entire embryo surface, thereby enclosing the larger, yolky macromeres (SS CASC, n.d.).
• Convergence and Extension: Cells undergoing involution also exhibit convergent extension, a movement where tissues narrow and lengthen, crucial for forming structures like the notochord (SS CASC, n.d.).
• Resulting Structures: The process culminates in the formation of the archenteron (gastrocoel) (Slideshare, n.d.; SS CASC, n.d.), the establishment of the three primary germ layers (ectoderm, mesoderm, and endoderm), and the temporary formation of the yolk plug, where yolky macromeres protrude from the blastopore (SS CASC, n.d.). The blastocoel cavity eventually diminishes and disappears (SS CASC, n.d.).
• Gastrulation in Chicks (Birds):
• Gastrulation in Chicks (Birds): Gastrulation in chick embryos occurs on a flat, disc-shaped embryo, known as the blastodisc, which sits atop a massive yolk (Bastiani, n.d.; Scribd, 2021). This process is uniquely characterized by the formation of the primitive streak (Synopsis IAS, 2024).
• Key Features: The primitive streak is central to establishing the embryo's axes and initiating cell movements (Synopsis IAS, 2024).

Gastrulation in Chick Embryo

• Cell Movements:
• Primitive Streak Formation: Gastrulation begins with the appearance of the primitive streak, a thickened linear band of cells within the epiblast, extending from the posterior to the anterior end of the embryo (Fiveable, n.d.; Gilbert, 2000; Synopsis IAS, 2024). This streak is crucial for establishing bilateral symmetry and defining the anterior-posterior axis of the embryo (Synopsis IAS, 2024).
• Ingression of Epiblast Cells: Epiblast cells actively migrate towards the primitive streak and then ingress, moving inward through it (Fiveable, n.d.; Gilbert, 2000; Synopsis IAS, 2024; Yang et al., 2002). During ingression, these cells undergo an epithelial-to-mesenchymal transition (EMT), shedding their epithelial characteristics and acquiring migratory mesenchymal properties (Synopsis IAS, 2024; Yang et al., 2002).
• Germ Layer Formation: The initial cells to ingress displace the hypoblast, forming the definitive endoderm, which will line the gut. Subsequent ingressing cells spread laterally to form the mesoderm (Synopsis IAS, 2024). The epiblast cells that do not ingress remain on the surface to form the ectoderm (Synopsis IAS, 2024).
• Role of Hensen's Node: A thickened area at the anterior extremity of the primitive streak, known as Hensen's node, functions as a critical organizer (Synopsis IAS, 2024). Cells that migrate through Hensen's node contribute to the formation of the notochord and head structures (Synopsis IAS, 2024). Signaling pathways, such as Wnt and BMP, are intricately involved in the differentiation of these germ layers, with their activities influenced by Hensen's node (Fiveable, n.d.; Synopsis IAS, 2024).
• Primitive Streak Regression: After reaching its maximal length, the primitive streak begins to regress posteriorly. As it regresses, the notochord and somites form in its wake, thereby establishing the axial body plan of the embryo (Synopsis IAS, 2024).
• Resulting Structures: The process leads to the formation of the three primary germ layers and the basic body plan, including the crucial notochord (Synopsis IAS, 2024).
• Gastrulation in Mice and Humans (Mammals):
• Gastrulation in Mice and Humans (Mammals): Despite the absence of a large yolk, mammalian gastrulation patterns strikingly resemble those observed in reptiles and birds, retaining the primitive streak mechanism (Gilbert, 2000). This remarkable similarity reflects a deep evolutionary conservation of developmental modules. Mammalian embryos obtain nutrients directly from the mother, which necessitates the development of specialized extraembryonic structures (Gilbert, 2000). The observation that mammalian gastrulation, despite the absence of a large yolk, largely mirrors that of birds and reptiles is a powerful testament to evolutionary conservation. Mammals evolved from reptilian ancestors, and the primitive streak mechanism, which is highly adapted for development on a large yolk, was an evolutionarily successful developmental module. Even when the selective pressure for a large yolk was removed due to the evolution of viviparity and direct maternal nutrient transfer, the underlying genetic and cellular program for gastrulation was largely retained. This suggests that fundamental developmental pathways can be remarkably robust and evolutionarily stable, even when the environmental context changes. The loss of yolk necessitated the evolution of new structures for nutrient acquisition, specifically the chorion and placenta (Gilbert, 2000). This illustrates how evolution often conserves core developmental processes while simultaneously innovating auxiliary structures to support them under novel environmental conditions. This phenomenon highlights the concept of "descent with modification" at the embryonic level, demonstrating that evolution frequently modifies existing structures and processes rather than inventing entirely new ones, leading to homologous developmental mechanisms across diverse species, even when their adult forms or reproductive strategies diverge significantly.
• Key Features: Mammalian development parallels that of reptiles and birds, with the inner cell mass conceptually resting on an imaginary yolk ball, following instructions inherited from reptilian ancestors (Gilbert, 2000).
• Cell Movements:
• Hypoblast and Epiblast Formation: The inner cell mass initially segregates into the hypoblast (primitive endoderm) and the epiblast (Gilbert, 2000). Hypoblast cells delaminate to form the extraembryonic endoderm, which gives rise to the yolk sac (Gilbert, 2000). The epiblast is believed to form the embryo proper (Gilbert, 2000).
• Amnionic Cavity Formation: The epiblast layer subsequently splits to form the amnionic cavity, which fills with fluid to provide protection and prevent desiccation of the developing embryo (Gilbert, 2000).
• Primitive Streak Formation and Ingression: Gastrulation initiates at the posterior end of the embryo with the formation of the primitive streak and node (Gilbert, 2000; Ichikawa et al., 2013). Epiblast cells migrate through the primitive streak, undergoing an epithelial-to-mesenchymal transition, losing E-cadherin, and detaching from their neighbors to ingress as individual cells (Gilbert, 2000; Ichikawa et al., 2013; "Ingression (biology)", n.d.).
• Germ Layer Formation: Cells ingressing through the primitive streak give rise to the definitive endoderm, which replaces the hypoblast cells, and the mesoderm (Gilbert, 2000). The remaining epiblast cells form the ectoderm (Gilbert, 2000).
• Notochord Formation: Cells migrating through the node contribute to the notochord. In mice, notochord cells are thought to integrate into the endoderm of the primitive gut before converging medially and folding off dorsally to form the notochord (Gilbert, 2000).
• Interkinetic Nuclear Migration (INM): Advanced live imaging studies have revealed that interkinetic nuclear migration (INM) occurs in the epiblast, where nuclei move between the apical and basal surfaces during the cell cycle (Ichikawa et al., 2013; Ichikawa et al., 2014). Furthermore, mesodermal cells migrate as individual entities rather than as a cohesive sheet (Ichikawa et al., 2013).
• Formation of Extraembryonic Structures:
• Trophoblast Derivatives: The original trophoblast cells divide to form the cytotrophoblast layer, while other cells undergo nuclear division without cytokinesis to form the multinucleated syncytiotrophoblast (Gilbert, 2000).
• Placenta Formation: The cytotrophoblast adheres to and penetrates the uterine wall, remodeling maternal blood vessels. The syncytiotrophoblast further invades uterine tissue (Gilbert, 2000). Shortly thereafter, mesodermal tissue extends from the gastrulating embryo to join the trophoblastic extensions, forming the blood vessels necessary for nutrient transport from the mother to the embryo (Gilbert, 2000). This complex of trophoblast tissue and blood vessel-containing mesoderm forms the chorion, which subsequently fuses with the uterine wall to create the placenta (Gilbert, 2000). The narrow connecting stalk of extraembryonic mesoderm that links the embryo to the trophoblast eventually develops into the vessels of the umbilical cord (Gilbert, 2000).

The table below provides a comparative overview of gastrulation patterns across different species:

Organism	Key Features of Gastrulation	Primary Cell Movements Involved	Unique Structures/Adaptations
Sea Urchin	Simple, elegant model; cells specified early in cleavage.	PMC Ingression (EMT), Vegetal Plate Invagination, Archenteron Elongation (filopodia, convergent extension).	Skeletogenic PMCs; Archenteron forms primitive gut. (Riesgo & Wessel, 2021)
Frog (Amphibian)	Influenced by large yolk; begins at gray crescent.	Invagination (bottle cells, blastopore), Involution (marginal zone cells), Epiboly (animal pole micromeres), Convergence, Extension.	Yolk plug; blastocoel disappears. (Slideshare, n.d.; SS CASC, n.d.)
Chick (Bird)	Occurs on flat blastodisc atop massive yolk; primitive streak formation is central.	Primitive Streak Formation, Epiblast Cell Ingression (EMT), Primitive Streak Regression.	Primitive streak, Hensen's node (organizer). (Synopsis IAS, 2024)
Mouse/Human (Mammal)	Resembles birds/reptiles despite no yolk; direct maternal nutrient acquisition.	Hypoblast/Epiblast Formation, Amnionic Cavity Formation, Primitive Streak Formation, Ingression, Interkinetic Nuclear Migration (INM).	Chorion, Placenta, Umbilical Cord; INM in epiblast. (Gilbert, 2000; Ichikawa et al., 2013; Ichikawa et al., 2014)

 

 Gastrulation in Frog Embryo

Summery in Video Link: 

Click the Link 👉 Gastrulation: The Fundamental Stages of Early Embryonic Development

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