Bird Embryology: From Egg to Chick – Segmentation, Blastula & Gastrulation
Bird Embryology: From Egg to Chick
A. Fertilization and Egg Formation
After ovulation, the egg travels through the oviduct, where fertilization occurs. Glands in the oviduct secrete albumin, forming the egg white. The egg’s twisting journey through the oviduct’s spiral folds creates the chalazae, which hold the yolk in place. Shell membranes and the calcareous shell are also added during this process. The egg then enters the cloaca and is laid. This entire process takes approximately 16 to 20 hours.
B. Segmentation
The egg undergoes meroblastic segmentation, meaning the yolk’s mass restricts cell division to a small area called the blastodisc at the animal pole. Segmentation begins with a vertical groove in the blastodisc, followed by a perpendicular groove. Further divisions create numerous cells, forming the blastula and blastocoel.
C. The Blastula
Initially, the blastomeres resemble a blackberry (morula stage). They then arrange around a central cavity (blastocoel), forming the blastula. While the overall size doesn’t change significantly, the number of cells and genetic material increases.
Primary Blastula
Around the 32-64 cell stage, a new type of division occurs, creating a lower layer of blastomeres separated from the yolk. Horizontal divisions form the primary blastocoel. The blastoderm now has two regions: the area pellucida (lighter central part) and the area opaca (peripheral layers).
Secondary Blastula
The blastoderm in the area pellucida becomes a two-layered structure (diploblastic). The outer layer is the ectoderm, and the inner layer is the endoderm. The primary blastocoel divides into the archenteron (above the yolk) and the secondary blastocoel (between the ectoderm and endoderm).
D. Gastrulation
Gastrulation begins in the early hours of incubation. The blastoderm thickens, and a primitive streak forms. This process leads to the differentiation of the three germ layers: ectoderm, mesoderm, and endoderm. Gastrulation is complete after 18-20 hours of incubation, and neurulation begins, leading to the formation of the nervous system and other organs.
E. Birth of Birds
Incubation duration varies depending on the bird species. The chick uses an”egg toot” on its beak to break the shell. Chicks can be nidifugous (able to walk and feed themselves soon after hatching) or nidicolous (requiring parental care).
Steps of Hatching:
- Preparation for Birth: The chick positions itself with its beak towards the air cell and pierces it.
- Rupture of the Shell: The chick uses its egg tooth and neck muscles to crack the shell.
- The Crack Takes Shape: The chick continues to peck and rotate, enlarging the crack.
- The Fissure Becomes a Crack Around the Egg: The crack extends around the circumference of the egg.
- The Chick Secures its Position: The chick grips the shell’s edge with its toes.
- The Birth Itself: The chick pushes with its legs and shoulders, emerging from the egg.
- Life in the Outside World: The chick begins its life outside the egg.