Biology Quiz on Twins and Gastrulation

Study Notes

Sheep, goats, dogs, cats and pigs usually give birth to multiple offspring.
Monozygotic (Identical) twins: Develop from a single zygote. Are genetically identical. Share a common placenta.
Dizygotic twins: Develop from two fertilized ova. Each twin has its own placenta. May be of different sex and morphological features.
Monsters or united twins: Incompletely separated monozygotic twins. Can be attached at the head (craniopagus), thorax (thoracopagus), or sacrum (pygopagus).

The transformation of the blastula to gastrula by forming three germ layers: ectoderm, mesoderm, and endoderm.
Occurs in the inner cell mass of mammalian ova or the blastoderm (area pellucida) of avian ova.
Delamination: Separation of the lower layer of the germinal disc to form the hypoblast (endoderm). The remaining germinal disc becomes the epiblast, which will form the mesoderm and ectoderm.
Involution: Migration of superficial epiblast cells through the primitive streak to a new position between the endoderm and the outer epiblast layer. These migrating cells become the mesoderm, and the outer epiblast layer becomes the ectoderm.
Mesoderm subdivisions:
- Paraxial mesoderm (somites): Lateral to the neural tube.
- Intermediate mesoderm:
- Lateral mesoderm: Divided by the coelomic cavity.
  - Parietal or somatic mesoderm: Outer layer.
  - Visceral or splanchnic mesoderm: Inner layer.

Central and peripheral nervous systems (brain and spinal cord)
Nerve cells of sense organs (eye and ear)
Epidermis of the skin, hair, nails, sweat and sebaceous glands, mammary glands
Mucous membrane lining the oral cavity, gums, enamel of teeth, salivary glands, pituitary gland, epithelium of the nasal cavity and paranasal sinuses
External auditory meatus and the outer layer of the tympanic membrane

Most of the digestive and respiratory tracts, urinary bladder and urethra
Epithelial lining of the tympanic cavity and auditory (Eustachian) tube
Epithelial cells of tonsils, thyroid and parathyroid, thymus, liver, pancreas, and prostate gland

Paraxial mesoderm: Forms vertebrae and associated muscles.
Intermediate mesoderm: Forms most of the urogenital system.
Lateral mesoderm: Forms bones, muscles, and blood vessels of the body wall and internal organs.

Gastrulation occurs in two steps, ** involution ** and ** delamination.
The first step of gastrulation is ** delamination.
The process of formation of ectoderm, mesoderm, and endoderm is called ** gastrulation.
The second step of gastrulation is ** involution.

(d) It ends by formation of bilaminar embryo formed of epiplast and hypoblast.
- Correct statement: Gastrulation ends with the formation of a trilaminar embryo (ectoderm, mesoderm, and endoderm).

Gastrulation means the transformation of ** blastula ** to ** gastrula ** by formation of ** three germ layers.
The kidney is derived from ** intermediate mesoderm.
Brain is derived from ** ectoderm.
Vertebrae are derived from ** paraxial mesoderm.
Epithelial lining the digestive system is derived from ** endoderm.