Question

Which of the following describes a blastula?

• A. A triple-layered structure that has folded on itself with a blastopore as its opening.
• B. A solid ball of cells composed of three germ layers.
• C. A hollow ball of cells with a fluid-filled cavity called a blastocoel.
• D. A fertilized egg.
• E. An embryo with a folded neural crest.

Answer 1

Answer: (C)

A hollow ball of cells with a fluid-filled cavity called a blastocoel.

About 30-36 hours of fertilisation, the zygote moves through the isthmus of oviduct to uterus and divides by mitosis. This process is called cleavage and it results in the formation of two smaller cells called blastomeres. After further two cleavages, a ball of 8 cells called morula is formed and cleavage continues. Morula stage contains 16 cells (occasionally 32). Morula undergoes compaction and forms an outer mass of smaller cells with tight junctions and an inner mass of cells with gap junctions. After morula, a 64-celled stage is formed which is called blastula. Blastula contains a fluid filled cavity called blastocoel. This embryo is called blastocyst.

So, the correct answer that describes a blastula is 'A hollow ball of cells with a fluid-filled cavity called a blastocoel'.