Fertilization, Formation and Foetal Membrane in Mammals | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

1. Fertilization Process:

   • The head of the sperm enters the egg, and this joining is called fertilization.
   • After entering, the sperm head rotates and moves toward the center of the egg.

2. Chromosomal Changes:

   • The nuclei (containing genetic material) of both the egg and sperm come together.
   • The protective membranes around these nuclei disappear, and the genetic material combines to form chromosomes.

3. Cell Division:

   • The centrosome of the sperm divides into two and positions itself between the newly formed chromosome groups.
   • This marks the start of cell division.

4. Result of Fertilization:

   • The original number of chromosomes, which is unique to each species, is restored through fertilization.

1. Cleavage Process in Mammals:

   • Cleavage is the process where the fertilized egg (zygote) divides into smaller cells, known as blastomeres.
   • This cell division continues until a certain number of cells are formed, creating a structure similar to the original zygote.

2. Formation of Blastula:

   • Blastomeres arrange themselves in a circular manner, forming a structure called the blastula.
   • The blastula has a central space known as the blastocoel.

3. Structure of Blastula:

   • The blastula is a hollow sphere lined externally by blastomeres, which form the trophoblast.
   • Inside the blastula, there is an inner cell mass attached to the trophoblast, and this structure is called a blastocyst.

4. Development of Germ Layers:

   • The inner mass separates into an outer layer called ectoderm and an inner layer called endoderm.
   • The endoderm forms a roof for the yolk sac, and together, these layers are referred to as the embryonic disc.

5. Primitive Streak and Mesoderm Formation:

   • A primitive streak appears, and cell proliferation in this area produces cells of the mesoderm.
   • The mesoderm layer differentiates into the notochord and two lateral sheets that form the somatic and splanchnic layers.

6. Formation of Body Cavity:

   • The somatic mesoderm and ectoderm together create the body wall, known as Somatopleurae.
   • The endoderm and splanchnic mesoderm together form the gut wall, known as Splanchnopleurae.

Fig: Formation of germ layers in mammals

Foetal Membranes and Placentation

1. Fetal Membranes and Appendages:

   • The set of structures surrounding and supporting the developing fetus include the yolk sac, chorion, allantois, and amnion.
   • The yolk sac, although not functional in mammals, is important as it's where blood cells first appear.

2. Amnion's Protective Role:

   • The amnion is like a protective bag that surrounds and suspends the embryo.
   • It keeps the embryo safe from drying out and helps maintain its shape without distortion.

3. Muscle Fibers and Contractions:

   • Muscles in the mesodermal layer of the embryo develop and contract rhythmically.
   • These contractions gently move the embryo, preventing it from sticking to the surroundings.

4. Allantois as a Temporary Storage:

   • The allantois serves as a temporary sac for storing the fetus's urine.

5. Placenta's Vital Role:

   • The chorion, covered in vascular villi, interacts closely with the endometrium (lining of the uterus).
   • This interaction forms the placenta, which is crucial for nutrition, respiration, waste elimination, and protection of the developing embryo.

Fig: Formation of octal membranes