Is the chromosome number of zygotes, embryonal cells and adult of a particular organism always constant? How is the constancy maintained in these three stages?

Explanation

Yes, zygote, embryonic cell and adult of a particular organism always constant have constant chromosome number.
Sexual reproduction includes gamete formation through meiosis and fertilization. Meiosis reduces the number of chromosomes to half in male and female gametes. This reduced chromosome number is then restored to normal during fertilization of male and female gametes. This is how, constant chromosome number is maintained in sexually reproducing organisms. Growth and development of zygote into embryonic cell and then into adult one takes place by mitosis (equatorial cell division) which produce the daughter cells carrying same chromosome number as that of parent cell.