Question

When a bird lays a fertilised egg, the embryo is in:
A) Zygote stage
B) Blastula stage     
C) Early gastrula stage
D) Complete gastrula stage

Answer 1

Hint: Early multicellular embryo is made up of two or more germinal cell layers, from which the different organs eventually develop. The hollow, one-layered ball of cells known as a blastula, which is produced by the recurrent cell division, or cleavage, of a fertilised egg, gives rise to the gastrula.

Step by step solution:
When an embryo transitions from a one-dimensional layer of epithelial cells (blastula) to a multilayered, multidimensional structure known as the gastrula, it goes through a youthful developmental process called gastrulation.
The blastula stage is the first stage of animal embryonic development. It is created by the cleavage of a fertilised ovum and has a central cavity filled with fluid termed the blastocoel surrounded by a spherical layer of about 128 cells.
During the third week of human development, there is gastrulation. One of the most important stages of development, gastrulation produces the three primary germ layers (ectoderm, endoderm, and mesoderm) and primes the system for organogenesis.

So, option C is correct.

Note: Gastrulation is the process through which the three layers of embryonic tissue, or germ layers, are formed. These cells will multiply, move, and differentiate during development into the four main adult tissues: epithelial tissue, connective tissue, muscle tissue, and nerve tissue.
Neurulation, which takes place in the third and fourth weeks following fertilisation, is the embryo's next significant development after gastrulation. This is the process by which the embryo creates the building blocks of the nervous system.