Question

Spreading of presumptive ectoderm on underlying mass of cells is?
A. Epiboly
B. Invagination
C. Ingression
D. Delamination

Answer 1

Hint: In the early phase of embryonic development, the blastula gets reorganized into a multilayered structure known as gastrula in the gastrulation phase. The embryo establishes distinct cell lineages and its basic axes of the body is set.

Complete Answer:
There are five basic types of cell movements that occur during gastrulation in the embryo: Invagination, involution, ingression, delamination, and epiboly.
- Invagination is a type of cell movement that occurs only in the vegetal pole and involves folding of an area of the exterior of the sheet towards the inside of the blastula. It helps in the establishment of the body plan,
- Involution is defined as the shrinking of the organ to its original size.
- Ingression involves the formation of mesenchyme cells in the embryo at the onset of gastrulation.
- Epiboly is the movement which involves the spreading of the epithelial layers of cells. The spreading of presumptive ectoderm on the underlying mass of cells occurs during this movement.
- Delamination involves splitting or migration of one sheet into two sheets.
Due to all these types of movements, the primary germ layers of the embryo- ectoderm, mesoderm, and endoderm are formed. This basic mechanism of gastrulation is similar in all animals.

Hence the correct answer is option A.

Note: The human embryo development has the following sequences of development- fertilization, cleavage, blastulation, implantation, embryonic disc formation, gastrulation, neurulation, development of organs and organ system and development of physical features. It all occurs during the nine month of gestation period in human females.