The formation of archenteron starts in 

The correct answer is 'Blastula'.

Key Points

• Blastula:
  • The blastula is an early stage of embryonic development in animals, following the morula stage and preceding the gastrula stage.
  • It is characterized by the presence of a hollow cavity called the blastocoel, surrounded by a layer of cells called blastomeres.
  • The formation of the archenteron, the primitive gut, begins during the blastula stage as cells start to migrate and differentiate to form the three primary germ layers.

Additional Information

• Morula:
  • The morula is an early stage in embryonic development, consisting of a solid ball of cells resulting from the division of the fertilized egg. It precedes the blastula stage and does not have a cavity or differentiated layers yet.
• Early neurula:
  • The early neurula stage follows gastrulation and involves the formation of the neural tube, which will eventually develop into the central nervous system. The archenteron formation is already completed by this stage.
• Early gastrula:
  • The early gastrula stage follows the blastula and is characterized by the movement of cells to form the three germ layers (ectoderm, mesoderm, and endoderm). The archenteron is further developed during this stage, but its initiation starts in the blastula stage.