Question

The transformation of the blastula into the gastrula is characterised by the formation of three germ layers. Which of the following is responsible for the formation of these germ layers?

• A. Corona radiata of the zygote
• B. Zona pellucida of the blastocyst
• C. Trophoblast of the blastocyst
• D. Inner cell mass of the blastocyst

Answer 1

The correct option is D Inner cell mass of the blastocyst

The zygote undergoes cleavage divisions to form cells called blastomeres. The embryo with 8-16 cells is called the morula. The morula continues to divide and transforms into a blastocyst. As the blastocyst is formed, the zona pellucida becomes thinner and finally disappears.
The blastocyst has two types of cells- the outer trophoblast and inner cell mass or embryoblast.
The inner cell mass or embryoblast further undergoes division and gives rise to the three germinal layers.
The layer of granulosa cells lying immediately outside the zona pellucida layer of the oocyte is called corona radiata. After fertilisation (fusion of the male and female gamete), when the formed zygote (fertilised ovum) starts moving towards the isthmus of the oviduct, it loses the layer of corona radiata. So, in the further stages of development of the zygote, the corona radiata layer is absent altogether.