Question

Mesoderm is formed through invagination of

• A. Ectoderm
• B. Endoderm
• C. Inner mass of cells
• D. Primitive streak

Answer 1

Answer: (D)

Primitive streak

• In the blastula stage, there is the formation of the primitive streak. It is the slight invagination of the inner cell mass and thus forms a bilaminar layer.
  
• The cells of the primitive streak move in between the layers and form the mesoderm.
  
• Thus primitive streak establishes bilateral symmetry in animals.
  

So, the correct option is 'Primitive streak'.