Events in the diagram are (in sequential order)

• A
  Fission of gametes $\rightarrow$ new individual $\rightarrow$ zygote
• B
  Fusion of gametes $\rightarrow$ zygote $\rightarrow$ new individual (cell $2n$)
• C
  Fission of gametes $\rightarrow$ zygote $\rightarrow$ new individual (cell $2n$)
• D
  Stages in the gametogenesis