Question

Soon after the three germ layers are formed in a developing embryo, the process of organogenesis starts. The human brain is formed from the
a. Ectoderm
b. Endoderm
c. Mesoderm
d. Pertly endoderm and partly mesoderm

Answer 1

Hint: Organogenesis is the phase of embryonic development that starts after the formation of three germ layers. These three germ layers are formed from gastrulation and form the internal organs of the organism.

Complete answer:
Formation and development of the organs of an organism from embryonic cells is termed as organogenesis. Organogenesis involves the coordination of multiple developmental processes such as morphogenesis, cell proliferation and differentiation.
Germ layer is a primary layer of cells that forms during embryonic development. The presence of middle layer mesoderm in chordates makes them triploblastic animals. Germ layers give rise to all animal tissue and organs.

The three germ layers are:
> Ectoderm: It is the outermost layer and it gives rise to the central nervous system (CNS) that includes the brain and spinal cord and it also gives rise to peripheral nervous system.

> Mesoderm: It is the middle layer and gives rise to many internal organs such the muscles, spine and circulatory system. The mesoderm forms the lymphatic system, bone, cartilage and spleen.

> Endoderm: It is the innermost layer and it gives rise to the digestive system and other internal organs of the animals. The endoderm forms the pharynx, oesophagus, stomach, small intestine, colon and the liver.

Hence, the correct answer is option (A).

Note: Development of notochord is the primary step in the process of organogenesis which induces formation of neural plates. The development of the neural tube will give rise to the brain and spinal cord. The Lanugo is the soft hairy covering of the foetus. The neural crest is considered to be the fourth germ layer but it is derived from the ectoderm.