The transformation of an egg into a chick is one of nature's most remarkable processes. Within just three weeks of incubation, a chick emerges, fully formed, from its egg. This intricate development is difficult to fully comprehend without a background in embryology.
When an egg is laid, some initial embryonic development has already taken place. However, this development typically pauses until the environmental conditions are suitable for incubation. Initially, all the embryonic cells are identical, but as the chick embryo progresses, these cells begin to differentiate. Some cells will form vital organs, while others will develop into limbs like wings or legs.
As incubation begins, a distinct thickened layer of cells becomes visible at the tail end of the embryo, known as the primitive streak. This streak is the embryo's longitudinal axis, from which the head and backbone will develop. Concurrently, the precursor to the digestive tract begins to form, blood islands that will later evolve into the vascular system appear, and the eye starts to develop.
On the second day of incubation, these blood islands begin to interconnect, forming a vascular system while the heart starts developing in another area. By the 44th hour, the heart and vascular systems merge, and the heart starts beating. The embryo establishes two distinct circulatory systems: one for itself and another, the vitelline system, which extends into the egg.
By the end of the third day, the beak begins to form, and limb buds for the wings and legs become visible. During the fourth day, the chick embryo undergoes torsion and flexion, causing its body to rotate 90 degrees so that it lies with its left side on the yolk, forming a "C" shape. The mouth, tongue, and nasal pits start developing, forming parts of the digestive and respiratory systems. The heart, still visible outside the body, continues to enlarge. By the end of the fourth day, all the essential organs required for life after hatching are present, though the chick embryo is still indistinguishable from that of a mammal.
As the chick embryo grows, its development accelerates. By the seventh day, digits appear on the wings and feet, the heart is fully enclosed within the thoracic cavity, and the embryo begins to resemble a bird. By the tenth day, feathers and feather tracts are evident, and the beak begins to harden. On the fourteenth day, the claws start forming, and the embryo positions itself for hatching. After twenty days, the chick, now in the hatching position, pierces the air cell with its beak, initiating pulmonary respiration.
On the 21st day, the chick begins to hatch. It starts by pushing its beak through the air cell, signaling the drying up of the allantois, which served as its lungs during development. Using the egg tooth on its beak and the muscle on the back of its neck, the chick cuts through the shell. It alternates between resting and repositioning itself, continuing to cut until its head is free. Finally, the chick kicks free from the bottom of the shell, exhausted but gradually gaining strength as it dries and heals. The hatching process is complete, and the egg tooth will fall off within a few days.
Freshly Laid Hen’s Egg
The fully developed, freshly laid hen’s egg is notable for its large size, measuring approximately 3 cm in diameter and 5 cm in length. It contains a substantial amount of yolk, classifying it as a macrolecithal egg due to the abundance of yolk content. Oval in shape, the egg’s ovum is about 3 mm in diameter and contains a nucleus, which is surrounded by a layer of yolk-free cytoplasm located at the animal pole. The yolk is organized into concentric layers of alternating yellow and white sections, arranged around a flask-shaped structure known as the latebra. The neck of the latebra widens beneath the blastodisc to form the nucleus of Pander. The yellow coloration of the yolk is due to carotenoids, while the white layers are thinner compared to the thicker yellow layers. Comprising 49% water, the yolk also contains 33% phospholipids and 18% proteins, along with essential vitamins and carbohydrates, making it a vital nutrient source.
The ovum is encased by a lipoprotein layer known as the plasmalemma, which constitutes the egg’s plasma membrane. Surrounding the ovum are a series of egg membranes that contribute to its protection and structure.
Primary Membranes
The primary membranes are formed between the oocyte and the surrounding follicle and are secreted by follicle cells. The vitelline membrane, a significant component of the primary membrane, has a dual origin. The inner portion is produced by the ovary, while the outer portion is secreted by the fallopian tube, as noted by Balinsky.
Secondary Membranes
These membranes are secreted by the oviduct and overlay the vitelline membrane. Surrounding this membrane is the albumen, a protein-rich, water-containing substance commonly referred to as egg white. The albumen is divided into layers: the outermost layer, known as the thin albumen, is less dense, while the middle layer, called the thick albumen or dense albumen, is more viscous. The innermost layer is extremely thick and forms structures known as chalazae, which function as balancers, keeping the ovum centrally positioned within the egg.
Shell Membranes
Two additional membranes, referred to as shell membranes, encase the albumen. At the egg’s broader end, an air space forms between the two membranes. This air space develops when the egg cools from its original temperature of 60°C after being laid.
Shell
The outermost structure of the egg is the shell, a porous, calcareous layer that allows for gas exchange. In a freshly laid hen’s egg, the shell is soft but rapidly hardens upon cooling. The porous nature of the shell is essential for the embryo’s respiratory processes during development.
This complex structure of the chicken embryo in the egg provides a secure and nutrient-rich environment essential for chick embryo development, ensuring the embryo's growth and eventual hatching.
Stages in Chick Embryo Development
Before Egg Laying
- Fertilization
- Cellular division and growth
- Segregation of cells into specialized tissues
Between Laying and Incubation
- No growth occurs; this is a stage of inactive embryonic life.
During Incubation
- First Day
- 16 hours: First signs of resemblance to a chick embryo
- 18 hours: Formation of the alimentary tract
- 20 hours: Formation of the vertebral column
- 21 hours: Development of the nervous system begins
- 22 hours: Development of the head begins
- 24 hours: Formation of the eye begins
- Second Day
- 25 hours: Heart development begins
- 35 hours: Ear formation begins
- 42 hours: Heart begins to beat
- Third Day
- 60 hours: Formation of the nose begins
- 62 hours: Limb buds for legs appear
- 64 hours: Limb buds for wings appear
- Fourth Day: Tongue formation begins
- Fifth Day: Development of reproductive organs and sex differentiation
- Sixth Day: Beak formation begins
- Eighth Day: Feather development begins
- Tenth Day: Beak hardens
- Thirteenth Day: Scales and claws begin to form
- Fourteenth Day: The embryo positions itself for hatching
- Sixteenth Day: Scales, claws, and beak continue to harden
- Seventeenth Day: The beak turns towards the air cell
- Nineteenth Day: Yolk sac begins to enter the body cavity
- Twentieth Day: Yolk sac is fully drawn into the body cavity, and the embryo occupies most of the egg space except the air cell
- Twenty-first Day: Chick hatches
24-Hour Chick Embryo Development
At the 24-hour incubation stage, the chick embryo is characterized by its oval shape, with the primitive streak becoming visible. A key development during this period is the formation of the notochord, originating from Hensen's node, which serves as a foundational structure for the embryo’s axial development.
By this stage, the chicken embryo has formed four pairs of somites, which are segmented blocks of mesoderm critical to the development of the vertebral column and associated structures.
Neural Folds
The neural folds begin to develop on either side of the embryo's mid-dorsal line. These longitudinal folds of thickened ectoderm enclose a central space known as the neural groove, which is deeper in the anterior region and becomes shallower towards the posterior. Fusion of the folds begins near the first somite after 24 hours of incubation, while the posterior region remains unfused. This posterior region is referred to as the sinus rhomboidalis, where the regressing primitive streak continues its development.
Gut Formation
At this stage, the foregut is the only part of the digestive system that has begun to form, with both the midgut and hindgut yet to develop. The junction between the foregut and the future midgut is marked by the anterior intestinal portal, flanked by thickened splanchnic mesoderm.
Mesoderm
The mesoderm, a critical germ layer, lies between the ectoderm and endoderm and extends across the entire blastoderm. It will give rise to various structures such as muscles, bones, and the circulatory system.
Blood Islands
In the area opaca, the outer region surrounding the area pellucida, groups of mesenchymal cells, referred to as blood islands, begin to appear. These blood islands are precursors to the embryo’s blood vessels, forming the foundation of the embryonic circulatory system.
Area Vasculosa & Area Vitellina
Within the area opaca, the region containing blood islands is known as the area vasculosa, where blood vessel formation occurs. The outermost peripheral zone, devoid of blood vessels, is called the area vitellina, which serves as a boundary region for early embryonic development.
48-Hour Chick Embryo Development
At the 48-hour incubation stage, the chick embryo undergoes significant morphological changes, particularly in the head region, where rapid growth leads to a distinctive shape.
Flexure
In the head region, two distinct bends, known as flexures, become evident. The first, located at the midbrain, is termed the cranial flexure, which causes the forebrain and hindbrain to align parallel to each other. This bending is directed rightward and backward. Further down, in the posterior brain region, a cervical flexure forms, curving the head towards the right, eventually shaping the adult neck structure.
Torsion
By the 48-hour stage, the chicken embryo has undergone a process called torsion, where the anterior end twists towards the right, causing the embryo's front part to rest on the left side of the yolk. This rotation begins at the head and extends towards the cervical flexure, roughly at the level of the 13th somite.
Brain Development
The embryo’s brain is divided into three regions at this point: the forebrain, which differentiates into the telencephalon and diencephalon, and the hindbrain, which extends into the medulla oblongata.
Eye and Ear Formation
The eye development shows the formation of a two-layered optic cup, while the lens placode begins its growth. In the ear, the invagination of the otic vesicle is complete, and it connects to the ectoderm through the endolymphatic duct.
Alimentary Canal
The foregut has extended to a length of about 0.5 mm, signaling the early stages of the digestive system’s development.
Heart
At this stage, the heart takes on an S-shaped tubular structure. Although still in its primitive form, it has not yet developed distinct chambers.
Blood Vessels
Two ventral aortae emerge from the anterior end of the heart, extending backward and transitioning into the dorsal aortae to support the early circulatory system.
Kidney
The pronephros, an early kidney structure, is fully developed, while mesonephric tubules have begun to form, marking the start of the intermediate mesoderm’s involvement in renal development.
Somites
By the 48-hour mark, the embryo displays 25 pairs of somites, essential for the formation of the vertebral column and associated musculature.
96-Hour Chick Embryo Development
Fertilization in chickens occurs in the upper region of the hen's oviduct, where a male gamete penetrates and fertilizes the egg. Following fertilization, the chicken embryo begins its journey through the oviduct, a process that lasts around 22 hours. During this time, the early development of the chicken embryo takes place within the oviduct.
Cleavage Process
The cleavage in a chicken embryo in egg is limited to the blastodisc due to the large volume of yolk. This type of cleavage is known as meroblastic or discoidal cleavage.
- First Cleavage: Approximately five hours after fertilization, the first cleavage occurs, cutting through the center of the blastodisc. This cleavage is meridional and does not completely divide the blastodisc, so no distinct blastomeres are formed at this stage.
- Second Cleavage: Occurring at a right angle to the first cleavage, this division also fails to produce clear blastomeres.
- Third Cleavage: This cleavage is vertical and occurs on both sides of the first division, resulting in the formation of eight blastomeres. However, at this stage, these blastomeres lack distinct boundaries.
- Fourth Cleavage: The fourth cleavage results in the formation of eight central and eight peripheral blastomeres. At this point, distinct cells begin to emerge. The central cells are completely separated from the yolk.
After the fourth cleavage, subsequent divisions become irregular, leading to the formation of the blastoderm.
Formation of Blastocoel
As the central cells continue to develop, a horizontal cleft forms beneath them, creating an upper layer of cells and a segmentation cavity known as the blastocoel. Rapid cell division above the blastocoel results in a mass of cells arranged in several layers, each with clear boundaries. The cells at the periphery, known as marginal cells, remain attached to the yolk, forming a region called the zone of junction.
Area Pellucida and Area Opaca
The central cell mass of the blastoderm is lifted from the yolk and consists of four to five layers. This creates a transparent region called the area pellucida, which is free from yolk. Surrounding the area pellucida is the area opaca, where the cells are still in contact with the yolk, providing structural support to the developing chick embryo.
Gastrulation in Chick: Formation of Endoderm
The formation of the endoderm in a chicken embryo begins with the development of a single layer of cells within the blastocoel. Following this, the upper layer of cells is termed the epiblast. Several theories have been proposed to explain the formation of the endoderm.
- Infiltration Theory: Proposed by Peter in 1923, this theory suggests that cells in the blastoderm, rich in yolk, migrate into the blastocoel. This process starts at the posterior end of the blastoderm, with cells progressively moving forward to form the endoderm.
- Delamination Theory: Introduced by Spratt in 1946, this theory posits that the blastoderm consists of two or three layers. The lower layer separates from the upper layers through splitting, resulting in the formation of the endoderm (lower layer) and the ectoderm (upper layers). The blastocoel is situated between these two layers.
- Theory of Involution: Proposed by Peterson in 1909, this theory describes the formation of a slit-like opening at the posterior end of the blastoderm. Cells roll into the primary blastocoel through this opening, contributing to the formation of the endoderm.
- Theory of Invagination: Formulated by Jacobson in 1938, this theory suggests that the posterior end of the blastoderm invaginates into the blastocoel, creating a small pocket that eventually forms the endoderm.
Egg Laying and Incubation
The hen expels the egg from its cloaca between 9 A.M. and 3 P.M. By the time of laying, the endoderm formation is complete. For further development, the egg must be incubated. After laying, development pauses until the egg is kept at approximately 38°C. This temperature is maintained by the hen sitting on the egg. In artificial incubation, eggs are placed in incubators and require about 21 days to hatch.
Gastrulation in Chick: Formation of Primitive Streak & Mesoderm
In the chicken embryo development, the second phase of gastrulation involves the formation of the primitive streak. This structure appears as a thickened region along the mid-dorsal line of the posterior area pellucida and begins to form approximately eight hours after incubation starts. The thickening occurs due to the convergence of blastoderm cells towards the center. Initially, the primitive streak is short and broad but progressively extends towards the midpoint of the blastoderm. By eighteen to nineteen hours of incubation, the primitive streak is significantly developed, known as the definite primitive streak.
A narrow furrow, called the primitive groove, forms along the center of the primitive streak. The edges of this groove, known as primitive folds, are notably thickened. At the anterior end of the groove, a cluster of densely packed cells, referred to as Hensen's node or the primitive knob, is present. Within this node is a central pit called the primitive pit, which represents the remnant of the neurenteric canal.
As the primitive streak continues to elongate, the area pellucida also extends. During this elongation, cells from the primitive streak migrate into the space between the epiblast and hypoblast—a process termed immigration. The cells that immigrate develop into the prechordal plate, notochord, and mesoderm. An area anterior to the primitive streak where mesoderm cells do not migrate is known as the proamnion. This region will eventually give rise to the head of the embryo.
Gastrulation in Chick: Development of Mesoderm and Coelome
In the development of the chick embryo, the mesoderm originates from the primitive streak and forms as two distinct layers. Initially, an area devoid of mesoderm, known as the proamnion, is situated in front of the primitive streak. However, by 48 hours of incubation, mesoderm also occupies this region. The mesoderm differentiates into three types: dorsal, intermediate, and lateral mesoderm.
The notochordal cells organize to create a cylindrical structure called the notochordal process. This process initiates at Hensen's node and gradually elongates, leading to a reduction in the primitive streak over time. By the conclusion of gastrulation, the primitive streak diminishes and is incorporated into the tail bud.
The dorsal mesoderm, located flanking the notochord, is segmented into units known as somites. The first pair of somites appears approximately 21 hours after incubation, with an additional pair forming every hour thereafter. At 24 hours of incubation, the embryo contains four pairs of somites.
The intermediate mesoderm acts as a connecting stalk between the dorsal and lateral mesoderm and subsequently segments to form the kidneys. The lateral mesoderm extends around the periphery of the embryo and is classified into extra-embryonic and embryonic mesoderm. This lateral mesoderm divides into two layers: the somatic mesoderm, which is the upper layer, and the splanchnic mesoderm, which is the inner layer. The ectoderm and somatic mesoderm together are referred to as the somatopleure, while the splanchnic layer and endoderm constitute the splanchnopleure. The space between these two mesoderm layers is known as the coelom.
By the end of gastrulation, specific regions dedicated to organ formation begin to develop, marking a significant stage in chicken embryo development.
Fetal Membranes of the Chick Embryo
In amniotes, the developing embryo requires the formation of fetal membranes to support its growth and development. The presence of these membranes is crucial for the embryo's survival and proper development. If the number of membranes is limited, careful attention must be given to their management. Conversely, a greater number of membranes generally necessitates less intensive care.
In the chicken embryo, the development of these membranes originates from the initial blastoderm. The central part of the blastoderm will differentiate into the embryo itself, while the marginal blastoderm contributes to the formation of extra-embryonic membranes. Specifically, the amnion and chorion develop from the somatopleure, whereas the yolk sac and allantois arise from the splanchnopleure.
Amnion and Chorion
During embryonic development, the amnion and chorion are closely associated. The amnion serves as a protective sac surrounding the embryo, isolating it from the internal environment. It originates from somatopleuric amniotic folds, which include the head fold, lateral folds, and tail folds.
Approximately 30 hours into incubation, the head fold, known as the amniotic head fold, develops in front of the embryo's head. By the third day of incubation, the amniotic tail fold appears, growing in the opposite direction to the head fold. Concurrently, lateral folds form and grow dorsomedially. Eventually, the head fold, lateral folds, and tail fold converge near the posterior end of the embryo.
At 72 hours of incubation, these folds are still not fully fused, leaving an opening termed the amniotic umbilicus. Eventually, the folds unite, forming the sero-amniotic raphe at the point of union. This fusion creates the outer chorion and the inner amnion, derived from the somatopleure. In the chorion, the ectoderm is located on the outside and the mesoderm on the inside. Conversely, in the amnion, the ectoderm is on the inside and the mesoderm on the outside. The space between the amnion and chorion is known as the exocoel or extraembryonic coelom.
Functions of the Chorion
- The extraembryonic coelom, filled with fluid, provides protection to the developing embryo.
- This space also accommodates the developing allantois.
- The chorion integrates with the allantois to function as a respiratory organ.
Functions of the Amnion
- The amnion forms a sac around the embryo that contains amniotic fluid. This fluid protects the embryo from mechanical shocks and dehydration.
- It also simulates an aquatic environment, providing essential protection when the egg is laid.
Yolk Sac
The yolk sac emerges around 16 hours into incubation. It develops from the splanchnopleure, which includes both endoderm and mesoderm layers. Instead of forming a closed gut, the splanchnopleure expands over the yolk, ultimately becoming the yolk sac. The primitive gut is positioned above the yolk, and the yolk sac connects to the midgut through an opening.
Functions of the Yolk Sac
It digests the yolk, and the resulting nutrients are transported via the blood to the developing embryo, making the yolk sac a critical nutritive organ.
Allantois
The allantois forms from the ventral part of the caudal end of the hindgut, beginning its development on the third day of incubation. Originating from the splanchnopleure, the allantois grows rapidly to fill the entire exocoel. The mesoderm of the chorion fuses with that of the allantois, forming the chorioallantoic membrane. The allantois is connected to the hindgut by the allantoic stalk.
As the embryo grows, the allantoic and yolk stalks converge, and their mesodermal layers merge to form the umbilical stalk, covered by somatic umbilicus.
Functions of the Allantois
- The allantois, highly vascularized, acts as a respiratory organ.
- It stores nitrogenous waste products of the embryo.
- During later development, the allantoic circulation absorbs calcium from the eggshell, which is then used for bone formation in the embryo. This calcium absorption also thins the eggshell, aiding in the hatching process.