A fertilized egg is called a zygote, the first stage of embryonic development, formed after the fusion of the sperm and egg.  

- On Day 1 (J1) after fertilization, the embryo is in the zygote stage.  

- On Day 2 (J2), it reaches the four-cell stage.  

- On Day 3 (J3), it develops into an eight-cell embryo.  

- On Day 4 (J4), it forms a morula (a compact cluster of cells resembling a small mulberry).  

- On Day 5 (J5), it transitions into a blastocyst (also called a blastomere), a large structure eight times the size of the original egg, containing about 100 cells.