Question

What process is occurring as the fertilized egg becomes 2 cells?

what is the hollow ball stage of development called?

How do fertilization and early embryonic development differ in humans and sea urchin?

Answer 1

Answer. Cleavage is occuring as the fertilised egg or Zygote becomes 2-celled. Cleavage in human beings is the first stage of embryo genesis. First cleavage leads to the generation of 2 cells. Second cleavage produces 3 cells. Thereafter, morula stage comes.

Answer. Morula is a solid mass of cells after this blastula stage comes where a cavity is formed inside the embryo called blastocoel. The embryonal mass is confined to the anterior side. Implantation occurs in the late blastula stage. The embryo gets fixed into the uterine cavity by chorionic villi. Thereafter, gastrulation and embryo genesis occurs.

Answer. In human beings fertilisation occurs in the fallopian tube,followed by capacitation. Then acrosome reaction, cortical reaction, zona reaction, activation of an ovum and then karyogamy occurs.

Then Zygote divides by mitotic division in 30 hours to form 2 cells. The second division is at right angle to the first. It is on one side, forming 3 cells. Then 8-16 cells are formed or sometimes 32 cells too in the morula stage. Morula then undergoes cell transfer at the anterior side to form blastula, which forms gastrula later and organogenesis occurs.

In sea urchin, the events for fertilisation are -

• Acrosome reaction
• Sperm egg fusion
• Cortical reaction
• Formation of fertilisation coat which is absent in the human beings

Early embryonic reactions include the cleavage morula and the blastula stage.

In sea urchin cleavage results in the formation of 4 cells inspite of 3 in humans. Blastula stage is attained at 128 cells while in humans it is at 32 cells.